= Ruby Style Guide :idprefix: :idseparator: - :sectanchors: :sectlinks: :toc: preamble :toclevels: 1 ifndef::backend-pdf[] :toc-title: pass:[Table of Contents] endif::[] :source-highlighter: rouge

== Introduction

[quote, Officer Alex J. Murphy / RoboCop]

---

Role models are important.

---

ifdef::env-github[] TIP: You can find a beautiful version of this guide with much improved navigation at https://rubystyle.guide. endif::[]

This Ruby style guide recommends best practices so that real-world Ruby programmers can write code that can be maintained by other real-world Ruby programmers. A style guide that reflects real-world usage gets used, while a style guide that holds to an ideal that has been rejected by the people it is supposed to help risks not getting used at all - no matter how good it is.

The guide is separated into several sections of related guidelines. We've tried to add the rationale behind the guidelines (if it's omitted we've assumed it's pretty obvious).

We didn't come up with all the guidelines out of nowhere - they are mostly based on the professional experience of the editors, feedback and suggestions from members of the Ruby community and various highly regarded Ruby programming resources, such as https://pragprog.com/book/ruby4/programming-ruby-1-9-2-0["Programming Ruby"] and https://www.amazon.com/Ruby-Programming-Language-David-Flanagan/dp/0596516177["The Ruby Programming Language"].

This style guide evolves over time as additional conventions are identified and past conventions are rendered obsolete by changes in Ruby itself.

ifdef::env-github[] You can generate a PDF copy of this guide using https://asciidoctor.org/docs/asciidoctor-pdf/[AsciiDoctor PDF], and an HTML copy https://asciidoctor.org/docs/convert-documents/#converting-a-document-to-html[with] https://asciidoctor.org/#installation[AsciiDoctor] using the following commands:

[source,console]

Generates README.pdf

asciidoctor-pdf -a allow-uri-read README.adoc

Generates README.html

asciidoctor README.adoc

[TIP]

Install the rouge gem to get nice syntax highlighting in the generated document.

[source,console]

gem install rouge

==== endif::[]

[TIP]

If you're into Rails or RSpec you might want to check out the complementary https://github.com/rubocop/rails-style-guide[Ruby on Rails Style Guide] and https://github.com/rubocop/rspec-style-guide[RSpec Style Guide].

TIP: https://github.com/rubocop/rubocop[RuboCop] is a static code analyzer (linter) and formatter, based on this style guide.

=== Guiding Principles

[quote, Harold Abelson, Structure and Interpretation of Computer Programs]

---

Programs must be written for people to read, and only incidentally for machines to execute.

---

It's common knowledge that code is read much more often than it is written. The guidelines provided here are intended to improve the readability of code and make it consistent across the wide spectrum of Ruby code. They are also meant to reflect real-world usage of Ruby instead of a random ideal. When we had to choose between a very established practice and a subjectively better alternative we've opted to recommend the established practice.footnote:[Occasionally we might suggest to the reader to consider some alternatives, though.]

There are some areas in which there is no clear consensus in the Ruby community regarding a particular style (like string literal quoting, spacing inside hash literals, dot position in multi-line method chaining, etc.). In such scenarios all popular styles are acknowledged and it's up to you to pick one and apply it consistently.

Ruby had existed for over 15 years by the time the guide was created, and the language's flexibility and lack of common standards have contributed to the creation of numerous styles for just about everything. Rallying people around the cause of community standards took a lot of time and energy, and we still have a lot of ground to cover.

Ruby is famously optimized for programmer happiness. We'd like to believe that this guide is going to help you optimize for maximum programmer happiness.

=== A Note about Consistency

[quote, Ralph Waldo Emerson]

---

A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines.

---

A style guide is about consistency. Consistency with this style guide is important. Consistency within a project is more important. Consistency within one class or method is the most important.

However, know when to be inconsistent -- sometimes style guide recommendations just aren't applicable. When in doubt, use your best judgment. Look at other examples and decide what looks best. And don't hesitate to ask!

In particular: do not break backwards compatibility just to comply with this guide!

Some other good reasons to ignore a particular guideline:

• When applying the guideline would make the code less readable, even for someone who is used to reading code that follows this style guide.
• To be consistent with surrounding code that also breaks it (maybe for historic reasons) -- although this is also an opportunity to clean up someone else's mess (in true XP style).
• Because the code in question predates the introduction of the guideline and there is no other reason to be modifying that code.
• When the code needs to remain compatible with older versions of Ruby that don't support the feature recommended by the style guide.

=== Translations

Translations of the guide are available in the following languages:

• https://github.com/JuanitoFatas/ruby-style-guide/blob/master/README-zhCN.md[Chinese Simplified]
• https://github.com/JuanitoFatas/ruby-style-guide/blob/master/README-zhTW.md[Chinese Traditional]
• https://github.com/HassanTC/ruby-style-guide/blob/master/README-EgAr.md[Egyptian Arabic]
• https://github.com/gauthier-delacroix/ruby-style-guide/blob/master/README-frFR.md[French]
• https://github.com/fortissimo1997/ruby-style-guide/blob/japanese/README.ja.md[Japanese]
• https://github.com/dalzony/ruby-style-guide/blob/master/README-koKR.md[Korean]
• https://github.com/rubensmabueno/ruby-style-guide/blob/master/README-PT-BR.md[Portuguese (pt-BR)]
• https://github.com/arbox/ruby-style-guide/blob/master/README-ruRU.md[Russian]
• https://github.com/alemohamad/ruby-style-guide/blob/master/README-esLA.md[Spanish]
• https://github.com/CQBinh/ruby-style-guide/blob/master/README-viVN.md[Vietnamese]

NOTE: These translations are not maintained by our editor team, so their quality and level of completeness may vary. The translated versions of the guide often lag behind the upstream English version.

== Source Code Layout

[quote, Jerry Coffin (on indentation)]

---

Nearly everybody is convinced that every style but their own is ugly and unreadable. Leave out the "but their own" and they're probably right...

---

=== Source Encoding [[utf-8]]

Use UTF-8 as the source file encoding.

TIP: UTF-8 has been the default source file encoding since Ruby 2.0.

=== Tabs or Spaces? [[tabs-or-spaces]]

Use only spaces for indentation. No hard tabs.

=== Indentation [[spaces-indentation]]

Use two spaces per indentation level (aka soft tabs).

[source,ruby]

bad - four spaces

def some_method do_something end

good

def some_method do_something end

=== Maximum Line Length [[max-line-length]]

Limit lines to 80 characters.

TIP: Most editors and IDEs have configuration options to help you with that. They would typically highlight lines that exceed the length limit.

.Why Bother with 80 characters in a World of Modern Widescreen Displays?

---

A lot of people these days feel that a maximum line length of 80 characters is just a remnant of the past and makes little sense today. After all - modern displays can easily fit 200+ characters on a single line. Still, there are some important benefits to be gained from sticking to shorter lines of code.

First, and foremost - numerous studies have shown that humans read much faster vertically and very long lines of text impede the reading process. As noted earlier, one of the guiding principles of this style guide is to optimize the code we write for human consumption.

Additionally, limiting the required editor window width makes it possible to have several files open side-by-side, and works well when using code review tools that present the two versions in adjacent columns.

The default wrapping in most tools disrupts the visual structure of the code, making it more difficult to understand. The limits are chosen to avoid wrapping in editors with the window width set to 80, even if the tool places a marker glyph in the final column when wrapping lines. Some web based tools may not offer dynamic line wrapping at all.

Some teams strongly prefer a longer line length. For code maintained exclusively or primarily by a team that can reach agreement on this issue, it is okay to increase the line length limit up to 100 characters, or all the way up to 120 characters. Please, restrain the urge to go beyond 120 characters.

---

=== No Trailing Whitespace [[no-trailing-whitespace]]

Avoid trailing whitespace.

TIP: Most editors and IDEs have configuration options to visualize trailing whitespace and to remove it automatically on save.

=== Line Endings [[crlf]]

Use Unix-style line endings.footnote:[*BSD/Solaris/Linux/macOS users are covered by default, Windows users have to be extra careful.]

[TIP]

If you're using Git you might want to add the following configuration setting to protect your project from Windows line endings creeping in:

[source,console]

$ git config --global core.autocrlf true

====

=== Should I Terminate Files with a Newline? [[newline-eof]]

End each file with a newline.

TIP: This should be done via editor configuration, not manually.

=== Should I Terminate Expressions with ;? [[no-semicolon]]

Don't use ; to terminate statements and expressions.

[source,ruby]

bad

puts 'foobar'; # superfluous semicolon

good

puts 'foobar'

=== One Expression Per Line [[one-expression-per-line]]

Use one expression per line.

[source,ruby]

bad

puts 'foo'; puts 'bar' # two expressions on the same line

good

puts 'foo' puts 'bar'

puts 'foo', 'bar' # this applies to puts in particular

=== Operator Method Call

Avoid dot where not required for operator method calls.

[source,ruby]

bad

num.+ 42

good

num + 42

=== Spaces and Operators [[spaces-operators]]

Use spaces around operators, after commas, colons and semicolons. Whitespace might be (mostly) irrelevant to the Ruby interpreter, but its proper use is the key to writing easily readable code.

[source,ruby]

bad

sum=1+2 a,b=1,2 class FooError<StandardError;end

good

sum = 1 + 2 a, b = 1, 2 class FooError < StandardError; end

There are a few exceptions:

• Exponent operator:

[source,ruby]

bad

e = M * c ** 2

good

e = M * c**2

• Slash in rational literals:

[source,ruby]

bad

o_scale = 1 / 48r

good

o_scale = 1/48r

• Safe navigation operator:

[source,ruby]

bad

foo &. bar foo &.bar foo&. bar

good

foo&.bar

=== Safe navigation

Avoid chaining of &.. Replace with . and an explicit check. E.g. if users are guaranteed to have an address and addresses are guaranteed to have a zip code:

[source,ruby]

bad

user&.address&.zip

good

user && user.address.zip

If such a change introduces excessive conditional logic, consider other approaches, such as delegation: [source,ruby]

bad

user && user.address && user.address.zip

good

class User def zip address&.zip end end user&.zip

=== Spaces and Braces [[spaces-braces]]

No spaces after (, [ or before ], ). Use spaces around { and before }.

[source,ruby]

bad

some( arg ).other [ 1, 2, 3 ].each{|e| puts e}

good

some(arg).other [1, 2, 3].each { |e| puts e }

{ and } deserve a bit of clarification, since they are used for block and hash literals, as well as string interpolation.

For hash literals two styles are considered acceptable. The first variant is slightly more readable (and arguably more popular in the Ruby community in general). The second variant has the advantage of adding visual difference between block and hash literals. Whichever one you pick - apply it consistently.

[source,ruby]

good - space after { and before }

{ one: 1, two: 2 }

good - no space after { and before }

{one: 1, two: 2}

With interpolated expressions, there should be no padded-spacing inside the braces.

[source,ruby]

bad

"From: #{ user.first_name }, #{ user.last_name }"

good

"From: #{user.first_name}, #{user.last_name}"

=== No Space after Bang [[no-space-bang]]

No space after !.

[source,ruby]

bad

! something

good

!something

=== No Space inside Range Literals [[no-space-inside-range-literals]]

No space inside range literals.

[source,ruby]

bad

1 .. 3 'a' ... 'z'

good

1..3 'a'...'z'

=== Indent when to case [[indent-when-to-case]]

Indent when as deep as case.

[source,ruby]

bad

case when song.name == 'Misty' puts 'Not again!' when song.duration > 120 puts 'Too long!' when Time.now.hour > 21 puts "It's too late" else song.play end

good

case when song.name == 'Misty' puts 'Not again!' when song.duration > 120 puts 'Too long!' when Time.now.hour > 21 puts "It's too late" else song.play end

.A Bit of History

---

This is the style established in both "The Ruby Programming Language" and "Programming Ruby". Historically it is derived from the fact that case and switch statements are not blocks, hence should not be indented, and the when and else keywords are labels (compiled in the C language, they are literally labels for JMP calls).

---

=== Indent Conditional Assignment [[indent-conditional-assignment]]

When assigning the result of a conditional expression to a variable, preserve the usual alignment of its branches.

[source,ruby]

bad - pretty convoluted

kind = case year when 1850..1889 then 'Blues' when 1890..1909 then 'Ragtime' when 1910..1929 then 'New Orleans Jazz' when 1930..1939 then 'Swing' when 1940..1950 then 'Bebop' else 'Jazz' end

result = if some_cond calc_something else calc_something_else end

good - it's apparent what's going on

kind = case year when 1850..1889 then 'Blues' when 1890..1909 then 'Ragtime' when 1910..1929 then 'New Orleans Jazz' when 1930..1939 then 'Swing' when 1940..1950 then 'Bebop' else 'Jazz' end

result = if some_cond calc_something else calc_something_else end

good (and a bit more width efficient)

kind = case year when 1850..1889 then 'Blues' when 1890..1909 then 'Ragtime' when 1910..1929 then 'New Orleans Jazz' when 1930..1939 then 'Swing' when 1940..1950 then 'Bebop' else 'Jazz' end

result = if some_cond calc_something else calc_something_else end

=== Empty Lines between Methods [[empty-lines-between-methods]]

Use empty lines between method definitions and also to break up methods into logical paragraphs internally.

[source,ruby]

bad

def some_method data = initialize(options) data.manipulate! data.result end def some_other_method result end

good

def some_method data = initialize(options)

data.manipulate!

data.result end

def some_other_method result end

=== Two or More Empty Lines [[two-or-more-empty-lines]]

Don't use several empty lines in a row.

[source,ruby]

bad - It has two empty lines.

some_method

some_method

good

some_method

some_method

=== Empty Lines around Attribute Accessor [[empty-lines-around-attribute-accessor]]

Use empty lines around attribute accessor.

[source,ruby]

bad

class Foo attr_reader :foo def foo # do something... end end

good

class Foo attr_reader :foo

def foo # do something... end end

=== Empty Lines around Access Modifier [[empty-lines-around-access-modifier]]

Use empty lines around access modifier.

[source,ruby]

bad

class Foo def bar; end private def baz; end end

good

class Foo def bar; end

private

def baz; end end

=== Empty Lines around Bodies [[empty-lines-around-bodies]]

Don't use empty lines around method, class, module, block bodies.

[source,ruby]

bad

class Foo

def foo

begin

  do_something do

    something

  end

rescue

  something

end

true

end

end

good

class Foo def foo begin do_something do something end rescue something end end end

=== Trailing Comma in Method Arguments [[no-trailing-params-comma]]

Avoid comma after the last parameter in a method call, especially when the parameters are not on separate lines.

[source,ruby]

bad - easier to move/add/remove parameters, but still not preferred

some_method( size, count, color, )

bad

some_method(size, count, color, )

good

some_method(size, count, color)

=== Spaces around Equals [[spaces-around-equals]]

Use spaces around the = operator when assigning default values to method parameters:

[source,ruby]

bad

def some_method(arg1=:default, arg2=nil, arg3=[])

do something...

end

good

def some_method(arg1 = :default, arg2 = nil, arg3 = [])

do something...

end

While several Ruby books suggest the first style, the second is much more prominent in practice (and arguably a bit more readable).

=== Line Continuation in Expressions [[no-trailing-backslash]]

Avoid line continuation with \ where not required. In practice, avoid using line continuations for anything but string concatenation.

[source,ruby]

bad (\ is not needed here)

result = 1 -
2

bad (\ is required, but still ugly as hell)

result = 1
- 2

good

result = 1 - 2

long_string = 'First part of the long string'
' and second part of the long string'

=== Multi-line Method Chains [[consistent-multi-line-chains]]

Adopt a consistent multi-line method chaining style. There are two popular styles in the Ruby community, both of which are considered good - leading . and trailing ..

==== Leading . [[leading-dot-in-multi-line-chains]]

When continuing a chained method call on another line, keep the . on the second line.

[source,ruby]

bad - need to consult first line to understand second line

one.two.three. four

good - it's immediately clear what's going on the second line

one.two.three .four

==== Trailing . [[trailing-dot-in-multi-line-chains]]

When continuing a chained method call on another line, include the . on the first line to indicate that the expression continues.

[source,ruby]

bad - need to read ahead to the second line to know that the chain continues

one.two.three .four

good - it's immediately clear that the expression continues beyond the first line

one.two.three. four

A discussion on the merits of both alternative styles can be found https://github.com/rubocop/ruby-style-guide/pull/176[here].

=== Method Arguments Alignment [[no-double-indent]]

Align the arguments of a method call if they span more than one line. When aligning arguments is not appropriate due to line-length constraints, single indent for the lines after the first is also acceptable.

[source,ruby]

starting point (line is too long)

def send_mail(source) Mailer.deliver(to: '[email protected]', from: '[email protected]', subject: 'Important message', body: source.text) end

bad (double indent)

def send_mail(source) Mailer.deliver( to: '[email protected]', from: '[email protected]', subject: 'Important message', body: source.text) end

good

def send_mail(source) Mailer.deliver(to: '[email protected]', from: '[email protected]', subject: 'Important message', body: source.text) end

good (normal indent)

def send_mail(source) Mailer.deliver( to: '[email protected]', from: '[email protected]', subject: 'Important message', body: source.text ) end

=== Implicit Options Hash [[no-braces-opts-hash]]

IMPORTANT: As of Ruby 2.7 braces around an options hash are no longer optional.

Omit the outer braces around an implicit options hash.

[source,ruby]

bad

user.set({ name: 'John', age: 45, permissions: { read: true } })

good

user.set(name: 'John', age: 45, permissions: { read: true })

=== DSL Method Calls [[no-dsl-decorating]]

Omit both the outer braces and parentheses for methods that are part of an internal DSL (e.g., Rake, Rails, RSpec).

[source,ruby]

class Person < ActiveRecord::Base

bad

attr_reader(:name, :age)

good

attr_reader :name, :age

bad

validates(:name, { presence: true, length: { within: 1..10 } })

good

validates :name, presence: true, length: { within: 1..10 } end

=== Space in Method Calls [[parens-no-spaces]]

Do not put a space between a method name and the opening parenthesis.

[source,ruby]

bad

puts (x + y)

good

puts(x + y)

=== Space in Brackets Access

Do not put a space between a receiver name and the opening brackets.

[source,ruby]

bad

collection [index_or_key]

good

collection[index_or_key]

=== Multi-line Arrays Alignment [[align-multiline-arrays]]

Align the elements of array literals spanning multiple lines.

[source,ruby]

bad - single indent

menu_item = %w[Spam Spam Spam Spam Spam Spam Spam Spam Baked beans Spam Spam Spam Spam Spam]

good

menu_item = %w[ Spam Spam Spam Spam Spam Spam Spam Spam Baked beans Spam Spam Spam Spam Spam ]

good

menu_item = %w[Spam Spam Spam Spam Spam Spam Spam Spam Baked beans Spam Spam Spam Spam Spam]

== Naming Conventions

[quote, Phil Karlton]

---

The only real difficulties in programming are cache invalidation and naming things.

---

=== English for Identifiers [[english-identifiers]]

Name identifiers in English.

[source,ruby]

bad - identifier is a Bulgarian word, using non-ascii (Cyrillic) characters

заплата = 1_000

bad - identifier is a Bulgarian word, written with Latin letters (instead of Cyrillic)

zaplata = 1_000

good

salary = 1_000

=== Snake Case for Symbols, Methods and Variables [[snake-case-symbols-methods-vars]]

Use snake_case for symbols, methods and variables.

[source,ruby]

bad

:'some symbol' :SomeSymbol :someSymbol

someVar = 5

def someMethod

some code

end

def SomeMethod

some code

end

good

:some_symbol

some_var = 5

def some_method

some code

end

=== Identifiers with a Numeric Suffix [[snake-case-symbols-methods-vars-with-numbers]]

Do not separate numbers from letters on symbols, methods and variables.

[source,ruby]

bad

:some_sym_1

some_var_1 = 1

var_10 = 10

def some_method_1

some code

end

good

:some_sym1

some_var1 = 1

var10 = 10

def some_method1

some code

end

=== CapitalCase for Classes and Modules [[camelcase-classes]]

NOTE: CapitalCase is also known as UpperCamelCase, CapitalWords and PascalCase.

Use CapitalCase for classes and modules. (Keep acronyms like HTTP, RFC, XML uppercase).

[source,ruby]

bad

class Someclass

some code

end

class Some_Class

some code

end

class SomeXml

some code

end

class XmlSomething

some code

end

good

class SomeClass

some code

end

class SomeXML

some code

end

class XMLSomething

some code

end

=== Snake Case for Files [[snake-case-files]]

Use snake_case for naming files, e.g. hello_world.rb.

=== Snake Case for Directories [[snake-case-dirs]]

Use snake_case for naming directories, e.g. lib/hello_world/hello_world.rb.

=== One Class per File [[one-class-per-file]]

Aim to have just a single class/module per source file. Name the file name as the class/module, but replacing CapitalCase with snake_case.

=== Screaming Snake Case for Constants [[screaming-snake-case]]

Use SCREAMING_SNAKE_CASE for other constants (those that don't refer to classes and modules).

[source,ruby]

bad

SomeConst = 5

good

SOME_CONST = 5

=== Predicate Methods Suffix [[bool-methods-qmark]]

The names of predicate methods (methods that return a boolean value) should end in a question mark (i.e. Array#empty?). Methods that don't return a boolean, shouldn't end in a question mark.

[source,ruby]

bad

def even(value) end

good

def even?(value) end

=== Predicate Methods Prefix [[bool-methods-prefix]]

Avoid prefixing predicate methods with the auxiliary verbs such as is, does, or can. These words are redundant and inconsistent with the style of boolean methods in the Ruby core library, such as empty? and include?.

[source,ruby]

bad

class Person def is_tall? true end

def can_play_basketball? false end

def does_like_candy? true end end

good

class Person def tall? true end

def basketball_player? false end

def likes_candy? true end end

=== Dangerous Method Suffix [[dangerous-method-bang]]

The names of potentially dangerous methods (i.e. methods that modify self or the arguments, exit! (doesn't run the finalizers like exit does), etc) should end with an exclamation mark if there exists a safe version of that dangerous method.

[source,ruby]

bad - there is no matching 'safe' method

class Person def update! end end

good

class Person def update end end

good

class Person def update! end

def update end end

=== Relationship between Safe and Dangerous Methods [[safe-because-unsafe]]

Define the non-bang (safe) method in terms of the bang (dangerous) one if possible.

[source,ruby]

class Array def flatten_once! res = []

each do |e|
  [*e].each { |f| res << f }
end

replace(res)

end

def flatten_once dup.flatten_once! end end

=== Unused Variables Prefix [[underscore-unused-vars]]

Prefix with +_+ unused block parameters and local variables. It's also acceptable to use just +_+ (although it's a bit less descriptive). This convention is recognized by the Ruby interpreter and tools like RuboCop will suppress their unused variable warnings.

[source,ruby]

bad

result = hash.map { |k, v| v + 1 }

def something(x) unused_var, used_var = something_else(x)

some code

end

good

result = hash.map { |_k, v| v + 1 }

def something(x) _unused_var, used_var = something_else(x)

some code

end

good

result = hash.map { |_, v| v + 1 }

def something(x) _, used_var = something_else(x)

some code

end

=== other Parameter [[other-arg]]

When defining binary operators and operator-alike methods, name the parameter other for operators with "symmetrical" semantics of operands. Symmetrical semantics means both sides of the operator are typically of the same or coercible types.

Operators and operator-alike methods with symmetrical semantics (the parameter should be named other): +, -, +*+, /, %, **, ==, >, <, |, &, ^, eql?, equal?.

Operators with non-symmetrical semantics (the parameter should not be named other): <<, [] (collection/item relations between operands), === (pattern/matchable relations).

Note that the rule should be followed only if both sides of the operator have the same semantics. Prominent exception in Ruby core is, for example, Array#*(int).

[source,ruby]

good

def +(other)

body omitted

end

bad

def <<(other) @internal << other end

good

def <<(item) @internal << item end

bad

Returns some string multiplied other times

def *(other)

body omitted

end

good

Returns some string multiplied num times

def *(num)

body omitted

end

== Flow of Control

=== for Loops [[no-for-loops]]

Do not use for, unless you know exactly why. Most of the time iterators should be used instead. for is implemented in terms of each (so you're adding a level of indirection), but with a twist - for doesn't introduce a new scope (unlike each) and variables defined in its block will be visible outside it.

[source,ruby]

arr = [1, 2, 3]

bad

for elem in arr do puts elem end

note that elem is accessible outside of the for loop

elem # => 3

good

arr.each { |elem| puts elem }

elem is not accessible outside each block

elem # => NameError: undefined local variable or method `elem'

=== then in Multi-line Expression [[no-then]]

Do not use then for multi-line if/unless/when/in.

[source,ruby]

bad

if some_condition then

body omitted

end

bad

case foo when bar then

body omitted

end

bad

case expression in pattern then

body omitted

end

good

if some_condition

body omitted

end

good

case foo when bar

body omitted

end

good

case expression in pattern

body omitted

end

=== Condition Placement [[same-line-condition]]

Always put the condition on the same line as the if/unless in a multi-line conditional.

[source,ruby]

bad

if some_condition do_something do_something_else end

good

if some_condition do_something do_something_else end

=== Ternary Operator vs if [[ternary-operator]]

Prefer the ternary operator(?:) over if/then/else/end constructs. It's more common and obviously more concise.

[source,ruby]

bad

result = if some_condition then something else something_else end

good

result = some_condition ? something : something_else

=== Nested Ternary Operators [[no-nested-ternary]]

Use one expression per branch in a ternary operator. This also means that ternary operators must not be nested. Prefer if/else constructs in these cases.

[source,ruby]

bad

some_condition ? (nested_condition ? nested_something : nested_something_else) : something_else

good

if some_condition nested_condition ? nested_something : nested_something_else else something_else end

=== Semicolon in if [[no-semicolon-ifs]]

Do not use if x; .... Use the ternary operator instead.

[source,ruby]

bad

result = if some_condition; something else something_else end

good

result = some_condition ? something : something_else

=== case vs if-else [[case-vs-if-else]]

Prefer case over if-elsif when compared value is the same in each clause.

[source,ruby]

bad

if status == :active perform_action elsif status == :inactive || status == :hibernating check_timeout else final_action end

good

case status when :active perform_action when :inactive, :hibernating check_timeout else final_action end

=== Returning Result from if/case [[use-if-case-returns]]

Leverage the fact that if and case are expressions which return a result.

[source,ruby]

bad

if condition result = x else result = y end

good

result = if condition x else y end

=== One-line Cases [[one-line-cases]]

Use when x then ... for one-line cases.

NOTE: The alternative syntax when x: ... has been removed as of Ruby 1.9.

=== Semicolon in when [[no-when-semicolons]]

Do not use when x; .... See the previous rule.

=== Semicolon in in [[no-in-pattern-semicolons]]

Do not use in pattern; .... Use in pattern then ... for one-line in pattern branches.

[source,ruby]

bad

case expression in pattern; do_something end

good

case expression in pattern then do_something end

=== ! vs not [[bang-not-not]]

Use ! instead of not.

[source,ruby]

bad - parentheses are required because of op precedence

x = (not something)

good

x = !something

=== Double Negation [[no-bang-bang]]

Avoid unnecessary uses of !!

!! converts a value to boolean, but you don't need this explicit conversion in the condition of a control expression; using it only obscures your intention.

Consider using it only when there is a valid reason to restrict the result true or false. Examples include outputting to a particular format or API like JSON, or as the return value of a predicate? method. In these cases, also consider doing a nil check instead: !something.nil?.

[source,ruby]

bad

x = 'test'

obscure nil check

if !!x

body omitted

end

good

x = 'test' if x

body omitted

end

good

def named? !name.nil? end

good

def banned? !!banned_until&.future? end

=== and/or [[no-and-or-or]] [[and-or-flow]]

Do not use and and or in boolean context - and and or are control flow operators and should be used as such. They have very low precedence, and can be used as a short form of specifying flow sequences like "evaluate expression 1, and only if it is not successful (returned nil), evaluate expression 2". This is especially useful for raising errors or early return without breaking the reading flow.

[source,ruby]

good: and/or for control flow

x = extract_arguments or raise ArgumentError, "Not enough arguments!" user.suspended? and return :denied

bad

and/or in conditions (their precedence is low, might produce unexpected result)

if got_needed_arguments and arguments_valid

...body omitted

end

in logical expression calculation

ok = got_needed_arguments and arguments_valid

good

&&/|| in conditions

if got_needed_arguments && arguments_valid

...body omitted

end

in logical expression calculation

ok = got_needed_arguments && arguments_valid

bad

&&/|| for control flow (can lead to very surprising results)

x = extract_arguments || raise(ArgumentError, "Not enough arguments!")

Avoid several control flow operators in one expression, as that quickly becomes confusing:

[source,ruby]

bad

Did author mean conditional return because #log could result in nil?

...or was it just to have a smart one-liner?

x = extract_arguments and log("extracted") and return

good

If the intention was conditional return

x = extract_arguments if x return if log("extracted") end

If the intention was just "log, then return"

x = extract_arguments if x log("extracted") return end

NOTE: Whether organizing control flow with and and or is a good idea has been a controversial topic in the community for a long time. But if you do, prefer these operators over &&/||. As the different operators are meant to have different semantics that makes it easier to reason whether you're dealing with a logical expression (that will get reduced to a boolean value) or with flow of control.

.Why is using and and or as logical operators a bad idea?

---

Simply put - because they add some cognitive overhead, as they don't behave like similarly named logical operators in other languages.

First of all, and and or operators have lower precedence than the = operator, whereas the && and || operators have higher precedence than the = operator, based on order of operations.

[source,ruby]

foo = true and false # results in foo being equal to true. Equivalent to (foo = true) and false bar = false or true # results in bar being equal to false. Equivalent to (bar = false) or true

Also && has higher precedence than ||, where as and and or have the same one. Funny enough, even though and and or were inspired by Perl, they don't have different precedence in Perl.

[source,ruby]

true or true and false # => false (it's effectively (true or true) and false) true || true && false # => true (it's effectively true || (true && false) false or true and false # => false (it's effectively (false or true) and false) false || true && false # => false (it's effectively false || (true && false))

---

=== Multi-line Ternary Operator [[no-multiline-ternary]]

Avoid multi-line ?: (the ternary operator); use if/unless instead.

=== if as a Modifier [[if-as-a-modifier]]

Prefer modifier if/unless usage when you have a single-line body. Another good alternative is the usage of control flow and/or.

[source,ruby]

bad

if some_condition do_something end

good

do_something if some_condition

another good option

some_condition and do_something

=== Multi-line if Modifiers [[no-multiline-if-modifiers]]

Avoid modifier if/unless usage at the end of a non-trivial multi-line block.

[source,ruby]

bad

10.times do

multi-line body omitted

end if some_condition

good

if some_condition 10.times do # multi-line body omitted end end

=== Nested Modifiers [[no-nested-modifiers]]

Avoid nested modifier if/unless/while/until usage. Prefer &&/|| if appropriate.

[source,ruby]

bad

do_something if other_condition if some_condition

good

do_something if some_condition && other_condition

=== if vs unless [[unless-for-negatives]]

Prefer unless over if for negative conditions (or control flow ||).

[source,ruby]

bad

do_something if !some_condition

bad

do_something if not some_condition

good

do_something unless some_condition

another good option

some_condition || do_something

=== Using else with unless [[no-else-with-unless]]

Do not use unless with else. Rewrite these with the positive case first.

[source,ruby]

bad

unless success? puts 'failure' else puts 'success' end

good

if success? puts 'success' else puts 'failure' end

=== Parentheses around Condition [[no-parens-around-condition]]

Don't use parentheses around the condition of a control expression.

[source,ruby]

bad

if (x > 10)

body omitted

end

good

if x > 10

body omitted

end

NOTE: There is an exception to this rule, namely <<safe-assignment-in-condition,safe assignment in condition>>.

=== Multi-line while do [[no-multiline-while-do]]

Do not use while/until condition do for multi-line while/until.

[source,ruby]

bad

while x > 5 do

body omitted

end

until x > 5 do

body omitted

end

good

while x > 5

body omitted

end

until x > 5

body omitted

end

=== while as a Modifier [[while-as-a-modifier]]

Prefer modifier while/until usage when you have a single-line body.

[source,ruby]

bad

while some_condition do_something end

good

do_something while some_condition

=== while vs until [[until-for-negatives]]

Prefer until over while for negative conditions.

[source,ruby]

bad

do_something while !some_condition

good

do_something until some_condition

=== Infinite Loop [[infinite-loop]]

Use Kernel#loop instead of while/until when you need an infinite loop.

[source,ruby]

bad

while true do_something end

until false do_something end

good

loop do do_something end

=== loop with break [[loop-with-break]]

Use Kernel#loop with break rather than begin/end/until or begin/end/while for post-loop tests.

[source,ruby]

bad

begin puts val val += 1 end while val < 0

good

loop do puts val val += 1 break unless val < 0 end

=== Explicit return [[no-explicit-return]]

Avoid return where not required for flow of control.

[source,ruby]

bad

def some_method(some_arr) return some_arr.size end

good

def some_method(some_arr) some_arr.size end

=== Explicit self [[no-self-unless-required]]

Avoid self where not required. (It is only required when calling a self write accessor, methods named after reserved words, or overloadable operators.)

[source,ruby]

bad

def ready? if self.last_reviewed_at > self.last_updated_at self.worker.update(self.content, self.options) self.status = :in_progress end self.status == :verified end

good

def ready? if last_reviewed_at > last_updated_at worker.update(content, options) self.status = :in_progress end status == :verified end

=== Shadowing Methods [[no-shadowing]]

As a corollary, avoid shadowing methods with local variables unless they are both equivalent.

[source,ruby]

class Foo attr_accessor :options

ok

def initialize(options) self.options = options # both options and self.options are equivalent here end

bad

def do_something(options = {}) unless options[:when] == :later output(self.options[:message]) end end

good

def do_something(params = {}) unless params[:when] == :later output(options[:message]) end end end

=== Safe Assignment in Condition [[safe-assignment-in-condition]]

Don't use the return value of = (an assignment) in conditional expressions unless the assignment is wrapped in parentheses. This is a fairly popular idiom among Rubyists that's sometimes referred to as safe assignment in condition.

[source,ruby]

bad (+ a warning)

if v = array.grep(/foo/) do_something(v)

some code

end

good (MRI would still complain, but RuboCop won't)

if (v = array.grep(/foo/)) do_something(v)

some code

end

good

v = array.grep(/foo/) if v do_something(v)

some code

end

=== BEGIN Blocks [[no-BEGIN-blocks]]

Avoid the use of BEGIN blocks.

=== END Blocks [[no-END-blocks]]

Do not use END blocks. Use Kernel#at_exit instead.

[source,ruby]

bad

END { puts 'Goodbye!' }

good

at_exit { puts 'Goodbye!' }

=== Nested Conditionals [[no-nested-conditionals]]

Avoid use of nested conditionals for flow of control.

Prefer a guard clause when you can assert invalid data. A guard clause is a conditional statement at the top of a function that bails out as soon as it can.

[source,ruby]

bad

def compute_thing(thing) if thing[:foo] update_with_bar(thing[:foo]) if thing[:foo][:bar] partial_compute(thing) else re_compute(thing) end end end

good

def compute_thing(thing) return unless thing[:foo] update_with_bar(thing[:foo]) return re_compute(thing) unless thing[:foo][:bar] partial_compute(thing) end

Prefer next in loops instead of conditional blocks.

[source,ruby]

bad

[0, 1, 2, 3].each do |item| if item > 1 puts item end end

good

[0, 1, 2, 3].each do |item| next unless item > 1 puts item end

== Exceptions

=== raise vs fail [[prefer-raise-over-fail]]

Prefer raise over fail for exceptions.

[source,ruby]

bad

fail SomeException, 'message'

good

raise SomeException, 'message'

=== Raising Explicit RuntimeError [[no-explicit-runtimeerror]]

Don't specify RuntimeError explicitly in the two argument version of raise.

[source,ruby]

bad

raise RuntimeError, 'message'

good - signals a RuntimeError by default

raise 'message'

=== Exception Class Messages [[exception-class-messages]]

Prefer supplying an exception class and a message as two separate arguments to raise, instead of an exception instance.

[source,ruby]

bad

raise SomeException.new('message')

Note that there is no way to do raise SomeException.new('message'), backtrace.

good

raise SomeException, 'message'

Consistent with raise SomeException, 'message', backtrace.

---

=== return from ensure [[no-return-ensure]]

Do not return from an ensure block. If you explicitly return from a method inside an ensure block, the return will take precedence over any exception being raised, and the method will return as if no exception had been raised at all. In effect, the exception will be silently thrown away.

[source,ruby]

bad

def foo raise ensure return 'very bad idea' end

=== Implicit begin [[begin-implicit]]

Use implicit begin blocks where possible.

[source,ruby]

bad

def foo begin # main logic goes here rescue # failure handling goes here end end

good

def foo

main logic goes here

rescue

failure handling goes here

end

=== Contingency Methods [[contingency-methods]]

Mitigate the proliferation of begin blocks by using contingency methods (a term coined by Avdi Grimm).

[source,ruby]

bad

begin something_that_might_fail rescue IOError

handle IOError

end

begin something_else_that_might_fail rescue IOError

handle IOError

end

good

def with_io_error_handling yield rescue IOError

handle IOError

end

with_io_error_handling { something_that_might_fail }

with_io_error_handling { something_else_that_might_fail }

=== Suppressing Exceptions [[dont-hide-exceptions]]

Don't suppress exceptions.

[source,ruby]

bad

begin do_something # an exception occurs here rescue SomeError end

good

begin do_something # an exception occurs here rescue SomeError handle_exception end

good

begin do_something # an exception occurs here rescue SomeError

Notes on why exception handling is not performed

end

good

do_something rescue nil

=== Using rescue as a Modifier [[no-rescue-modifiers]]

Avoid using rescue in its modifier form.

[source,ruby]

bad - this catches exceptions of StandardError class and its descendant classes

read_file rescue handle_error($!)

good - this catches only the exceptions of Errno::ENOENT class and its descendant classes

def foo read_file rescue Errno::ENOENT => e handle_error(e) end

=== Using Exceptions for Flow of Control [[no-exceptional-flows]]

Don't use exceptions for flow of control.

[source,ruby]

bad

begin n / d rescue ZeroDivisionError puts 'Cannot divide by 0!' end

good

if d.zero? puts 'Cannot divide by 0!' else n / d end

=== Blind Rescues [[no-blind-rescues]]

Avoid rescuing the Exception class. This will trap signals and calls to exit, requiring you to kill -9 the process.

[source,ruby]

bad

begin

calls to exit and kill signals will be caught (except kill -9)

exit rescue Exception puts "you didn't really want to exit, right?"

exception handling

end

good

begin

a blind rescue rescues from StandardError, not Exception as many

programmers assume.

rescue => e

exception handling

end

also good

begin

an exception occurs here

rescue StandardError => e

exception handling

end

=== Exception Rescuing Ordering [[exception-ordering]]

Put more specific exceptions higher up the rescue chain, otherwise they'll never be rescued from.

[source,ruby]

bad

begin

some code

rescue StandardError => e

some handling

rescue IOError => e

some handling that will never be executed

end

good

begin

some code

rescue IOError => e

some handling

rescue StandardError => e

some handling

end

=== Reading from a file [[file-read]]

Use the convenience methods File.read or File.binread when only reading a file start to finish in a single operation.

[source,ruby]

text mode

bad (only when reading from beginning to end - modes: 'r', 'rt', 'r+', 'r+t')

File.open(filename).read File.open(filename, &:read) File.open(filename) { |f| f.read } File.open(filename) do |f| f.read end File.open(filename, 'r').read File.open(filename, 'r', &:read) File.open(filename, 'r') { |f| f.read } File.open(filename, 'r') do |f| f.read end

good

File.read(filename)

binary mode

bad (only when reading from beginning to end - modes: 'rb', 'r+b')

File.open(filename, 'rb').read File.open(filename, 'rb', &:read) File.open(filename, 'rb') { |f| f.read } File.open(filename, 'rb') do |f| f.read end

good

File.binread(filename)

=== Writing to a file [[file-write]]

Use the convenience methods File.write or File.binwrite when only opening a file to create / replace its content in a single operation.

[source,ruby]

text mode

bad (only truncating modes: 'w', 'wt', 'w+', 'w+t')

File.open(filename, 'w').write(content) File.open(filename, 'w') { |f| f.write(content) } File.open(filename, 'w') do |f| f.write(content) end

good

File.write(filename, content)

binary mode

bad (only truncating modes: 'wb', 'w+b')

File.open(filename, 'wb').write(content) File.open(filename, 'wb') { |f| f.write(content) } File.open(filename, 'wb') do |f| f.write(content) end

good

File.binwrite(filename, content)

=== Release External Resources [[release-resources]]

Release external resources obtained by your program in an ensure block.

[source,ruby]

f = File.open('testfile') begin

.. process

rescue

.. handle error

ensure f.close if f end

=== Auto-release External Resources [[auto-release-resources]]

Use versions of resource obtaining methods that do automatic resource cleanup when possible.

[source,ruby]

bad - you need to close the file descriptor explicitly

f = File.open('testfile')

some action on the file

f.close

good - the file descriptor is closed automatically

File.open('testfile') do |f|

some action on the file

end

=== Atomic File Operations [[atomic-file-operations]]

When doing file operations after confirming the existence check of a file, frequent parallel file operations may cause problems that are difficult to reproduce. Therefore, it is preferable to use atomic file operations.

[source,ruby]

bad - race condition with another process may result in an error in mkdir

unless Dir.exist?(path) FileUtils.mkdir(path) end

good - atomic and idempotent creation

FileUtils.mkdir_p(path)

bad - race condition with another process may result in an error in remove

if File.exist?(path) FileUtils.remove(path) end

good - atomic and idempotent removal

FileUtils.rm_f(path)

=== Standard Exceptions [[standard-exceptions]]

Prefer the use of exceptions from the standard library over introducing new exception classes.

== Assignment & Comparison

=== Parallel Assignment [[parallel-assignment]]

Avoid the use of parallel assignment for defining variables. Parallel assignment is allowed when it is the return of a method call (e.g. Hash#values_at), used with the splat operator, or when used to swap variable assignment. Parallel assignment is less readable than separate assignment.

[source,ruby]

bad

a, b, c, d = 'foo', 'bar', 'baz', 'foobar'

good

a = 'foo' b = 'bar' c = 'baz' d = 'foobar'

good - swapping variable assignment

Swapping variable assignment is a special case because it will allow you to

swap the values that are assigned to each variable.

a = 'foo' b = 'bar'

a, b = b, a puts a # => 'bar' puts b # => 'foo'

good - method return

def multi_return [1, 2] end

first, second = multi_return

good - use with splat

first, *list = [1, 2, 3, 4] # first => 1, list => [2, 3, 4]

hello_array = *'Hello' # => ["Hello"]

a = *(1..3) # => [1, 2, 3]

=== Values Swapping [[values-swapping]]

Use parallel assignment when swapping 2 values.

[source,ruby]

bad

tmp = x x = y y = tmp

good

x, y = y, x

=== Dealing with Trailing Underscore Variables in Destructuring Assignment [[trailing-underscore-variables]]

Avoid the use of unnecessary trailing underscore variables during parallel assignment. Named underscore variables are to be preferred over underscore variables because of the context that they provide. Trailing underscore variables are necessary when there is a splat variable defined on the left side of the assignment, and the splat variable is not an underscore.

[source,ruby]

bad

foo = 'one,two,three,four,five'

Unnecessary assignment that does not provide useful information

first, second, _ = foo.split(',') first, , _ = foo.split(',') first, * = foo.split(',')

good

foo = 'one,two,three,four,five'

The underscores are needed to show that you want all elements

except for the last number of underscore elements

*beginning, _ = foo.split(',') *beginning, something, _ = foo.split(',')

a, = foo.split(',') a, b, = foo.split(',')

Unnecessary assignment to an unused variable, but the assignment

provides us with useful information.

first, _second = foo.split(',') first, _second, = foo.split(',') first, *_ending = foo.split(',')

=== Self-assignment [[self-assignment]]

Use shorthand self assignment operators whenever applicable.

[source,ruby]

bad

x = x + y x = x * y x = x**y x = x / y x = x || y x = x && y

good

x += y x *= y x **= y x /= y x ||= y x &&= y

=== Conditional Variable Initialization Shorthand [[double-pipe-for-uninit]]

Use ||= to initialize variables only if they're not already initialized.

[source,ruby]

bad

name = name ? name : 'Bozhidar'

bad

name = 'Bozhidar' unless name

good - set name to 'Bozhidar', only if it's nil or false

name ||= 'Bozhidar'

[WARNING]

Don't use ||= to initialize boolean variables. (Consider what would happen if the current value happened to be false.)

[source,ruby]

bad - would set enabled to true even if it was false

enabled ||= true

good

enabled = true if enabled.nil?

====

=== Existence Check Shorthand [[double-amper-preprocess]]

Use &&= to preprocess variables that may or may not exist. Using &&= will change the value only if it exists, removing the need to check its existence with if.

[source,ruby]

bad

if something something = something.downcase end

bad

something = something ? something.downcase : nil

ok

something = something.downcase if something

good

something = something && something.downcase

better

something &&= something.downcase

=== Identity Comparison [[identity-comparison]]

Prefer equal? over == when comparing object_id. Object#equal? is provided to compare objects for identity, and in contrast Object#== is provided for the purpose of doing value comparison.

[source,ruby]

bad

foo.object_id == bar.object_id

good

foo.equal?(bar)

Similarly, prefer using Hash#compare_by_identity than using object_id for keys:

[source,ruby]

bad

hash = {} hash[foo.object_id] = :bar if hash.key?(baz.object_id) # ...

good

hash = {}.compare_by_identity hash[foo] = :bar if hash.key?(baz) # ...

Note that Set also has Set#compare_by_identity available.

=== Explicit Use of the Case Equality Operator [[no-case-equality]]

Avoid explicit use of the case equality operator ===. As its name implies it is meant to be used implicitly by case expressions and outside of them it yields some pretty confusing code.

[source,ruby]

bad

Array === something (1..100) === 7 /something/ === some_string

good

something.is_a?(Array) (1..100).include?(7) some_string.match?(/something/)

NOTE: With direct subclasses of BasicObject, using is_a? is not an option since BasicObject doesn't provide that method (it's defined in Object). In those rare cases it's OK to use ===.

=== is_a? vs kind_of? [[is-a-vs-kind-of]]

Prefer is_a? over kind_of?. The two methods are synonyms, but is_a? is the more commonly used name in the wild.

[source,ruby]

bad

something.kind_of?(Array)

good

something.is_a?(Array)

=== is_a? vs instance_of? [[is-a-vs-instance-of]]

Prefer is_a? over instance_of?.

While the two methods are similar, is_a? will consider the whole inheritance chain (superclasses and included modules), which is what you normally would want to do. instance_of?, on the other hand, only returns true if an object is an instance of that exact class you're checking for, not a subclass.

[source,ruby]

bad

something.instance_of?(Array)

good

something.is_a?(Array)

=== instance_of? vs class comparison [[instance-of-vs-class-comparison]]

Use Object#instance_of? instead of class comparison for equality.

[source,ruby]

bad

var.class == Date var.class.equal?(Date) var.class.eql?(Date) var.class.name == 'Date'

good

var.instance_of?(Date)

=== == vs eql? [[eql]]

Do not use eql? when using == will do. The stricter comparison semantics provided by eql? are rarely needed in practice.

[source,ruby]

bad - eql? is the same as == for strings

'ruby'.eql? some_str

good

'ruby' == some_str 1.0.eql? x # eql? makes sense here if want to differentiate between Integer and Float 1

== Blocks, Procs & Lambdas

=== Proc Application Shorthand [[single-action-blocks]]

Use the Proc call shorthand when the called method is the only operation of a block.

[source,ruby]

bad

names.map { |name| name.upcase }

good

names.map(&:upcase)

=== Single-line Blocks Delimiters [[single-line-blocks]]

Prefer {...} over do...end for single-line blocks. Avoid using {...} for multi-line blocks (multi-line chaining is always ugly). Always use do...end for "control flow" and "method definitions" (e.g. in Rakefiles and certain DSLs). Avoid do...end when chaining.

[source,ruby]

names = %w[Bozhidar Filipp Sarah]

bad

names.each do |name| puts name end

good

names.each { |name| puts name }

bad

names.select do |name| name.start_with?('S') end.map { |name| name.upcase }

good

names.select { |name| name.start_with?('S') }.map(&:upcase)

Some will argue that multi-line chaining would look OK with the use of {...}, but they should ask themselves - is this code really readable and can the blocks' contents be extracted into nifty methods?

=== Single-line do...end block [[single-line-do-end-block]]

Use multi-line do...end block instead of single-line do...end block.

[source,ruby]

bad

foo do |arg| bar(arg) end

good

foo do |arg| bar(arg) end

bad

->(arg) do bar(arg) end

good

->(arg) { bar(arg) }

=== Explicit Block Argument [[block-argument]]

Consider using explicit block argument to avoid writing block literal that just passes its arguments to another block.

[source,ruby]

require 'tempfile'

bad

def with_tmp_dir Dir.mktmpdir do |tmp_dir| Dir.chdir(tmp_dir) { |dir| yield dir } # block just passes arguments end end

good

def with_tmp_dir(&block) Dir.mktmpdir do |tmp_dir| Dir.chdir(tmp_dir, &block) end end

with_tmp_dir do |dir| puts "dir is accessible as a parameter and pwd is set: #{dir}" end

=== Trailing Comma in Block Parameters [[no-trailing-parameters-comma]]

Avoid comma after the last parameter in a block, except in cases where only a single argument is present and its removal would affect functionality (for instance, array destructuring).

[source,ruby]

bad - easier to move/add/remove parameters, but still not preferred

[[1, 2, 3], [4, 5, 6]].each do |a, b, c,| a + b + c end

good

[[1, 2, 3], [4, 5, 6]].each do |a, b, c| a + b + c end

bad

[[1, 2, 3], [4, 5, 6]].each { |a, b, c,| a + b + c }

good

[[1, 2, 3], [4, 5, 6]].each { |a, b, c| a + b + c }

good - this comma is meaningful for array destructuring

[[1, 2, 3], [4, 5, 6]].map { |a,| a }

=== Nested Method Definitions [[no-nested-methods]]

Do not use nested method definitions, use lambda instead. Nested method definitions actually produce methods in the same scope (e.g. class) as the outer method. Furthermore, the "nested method" will be redefined every time the method containing its definition is called.

[source,ruby]

bad

def foo(x) def bar(y) # body omitted end

bar(x) end

good - the same as the previous, but no bar redefinition on every foo call

def bar(y)

body omitted

end

def foo(x) bar(x) end

also good

def foo(x) bar = ->(y) { ... } bar.call(x) end

=== Multi-line Lambda Definition [[lambda-multi-line]]

Use the new lambda literal syntax for single-line body blocks. Use the lambda method for multi-line blocks.

[source,ruby]

bad

l = lambda { |a, b| a + b } l.call(1, 2)

correct, but looks extremely awkward

l = ->(a, b) do tmp = a * 7 tmp * b / 50 end

good

l = ->(a, b) { a + b } l.call(1, 2)

l = lambda do |a, b| tmp = a * 7 tmp * b / 50 end

=== Stabby Lambda Definition with Parameters [[stabby-lambda-with-args]]

Don't omit the parameter parentheses when defining a stabby lambda with parameters.

[source,ruby]

bad

l = ->x, y { something(x, y) }

good

l = ->(x, y) { something(x, y) }

=== Stabby Lambda Definition without Parameters [[stabby-lambda-no-args]]

Omit the parameter parentheses when defining a stabby lambda with no parameters.

[source,ruby]

bad

l = ->() { something }

good

l = -> { something }

=== proc vs Proc.new [[proc]]

Prefer proc over Proc.new.

[source,ruby]

bad

p = Proc.new { |n| puts n }

good

p = proc { |n| puts n }

=== Proc Call [[proc-call]]

Prefer proc.call() over proc[] or proc.() for both lambdas and procs.

[source,ruby]

bad - looks similar to Enumeration access

l = ->(v) { puts v } l[1]

bad - most compact form, but might be confusing for newcomers to Ruby

l = ->(v) { puts v } l.(1)

good - a bit verbose, but crystal clear

l = ->(v) { puts v } l.call(1)

== Methods

=== Short Methods [[short-methods]]

Avoid methods longer than 10 LOC (lines of code). Ideally, most methods will be shorter than 5 LOC. Empty lines do not contribute to the relevant LOC.

=== Top-Level Methods

Avoid top-level method definitions. Organize them in modules, classes or structs instead.

NOTE: It is fine to use top-level method definitions in scripts.

[source,ruby]

bad

def some_method; end

good

class SomeClass def some_method; end end

=== No Single-line Methods [[no-single-line-methods]]

Avoid single-line methods. Although they are somewhat popular in the wild, there are a few peculiarities about their definition syntax that make their use undesirable. At any rate - there should be no more than one expression in a single-line method.

NOTE: Ruby 3 introduced an alternative syntax for single-line method definitions, that's discussed in the next section of the guide.

[source,ruby]

bad

def too_much; something; something_else; end

okish - notice that the first ; is required

def no_braces_method; body end

okish - notice that the second ; is optional

def no_braces_method; body; end

okish - valid syntax, but no ; makes it kind of hard to read

def some_method() body end

good

def some_method body end

One exception to the rule are empty-body methods.

[source,ruby]

good

def no_op; end

=== Endless Methods

Only use Ruby 3.0's endless method definitions with a single line body. Ideally, such method definitions should be both simple (a single expression) and free of side effects.

NOTE: It's important to understand that this guideline doesn't contradict the previous one. We still caution against the use of single-line method definitions, but if such methods are to be used, prefer endless methods.

[source,ruby]

bad

def fib(x) = if x < 2 x else fib(x - 1) + fib(x - 2) end

good

def the_answer = 42 def get_x = @x def square(x) = x * x

Not (so) good: has side effect

def set_x(x) = (@x = x) def print_foo = puts("foo")

=== Ambiguous Endless Method Definitions [[ambiguous-endless-method-defintions]]

Keywords with lower precedence than = can appear ambiguous when used after an endless method definition. This includes and, or, and the modifier forms of if, unless, while, and until. In these cases, the code may appear to include these keywords as part of the method body, but instead they actually modify the method definition itself.

In this cases, prefer using a normal method over an endless method.

[source,ruby]

bad

def foo = true if bar

good - using a non-endless method is more explicit

def foo true end if bar

ok - method body is explicit

def foo = (true if bar)

ok - method definition is explicit

(def foo = true) if bar

=== Double Colons [[double-colons]]

Use :: only to reference constants (this includes classes and modules) and constructors (like Array() or Nokogiri::HTML()). Do not use :: for regular method calls.

[source,ruby]

bad

SomeClass::some_method some_object::some_method

good

SomeClass.some_method some_object.some_method SomeModule::SomeClass::SOME_CONST SomeModule::SomeClass()

=== Colon Method Definition [[colon-method-definition]]

Do not use :: to define class methods.

[source,ruby]

bad

class Foo def self::some_method end end

good

class Foo def self.some_method end end

=== Method Definition Parentheses [[method-parens]]

Use def with parentheses when there are parameters. Omit the parentheses when the method doesn't accept any parameters.

[source,ruby]

bad

def some_method()

body omitted

end

good

def some_method

body omitted

end

bad

def some_method_with_parameters param1, param2

body omitted

end

good

def some_method_with_parameters(param1, param2)

body omitted

end

=== Method Call Parentheses [[method-invocation-parens]][[method-call-parens]]

Use parentheses around the arguments of method calls, especially if the first argument begins with an open parenthesis (, as in f((3 + 2) + 1).

[source,ruby]

bad

x = Math.sin y

good

x = Math.sin(y)

bad

array.delete e

good

array.delete(e)

bad

temperance = Person.new 'Temperance', 30

good

temperance = Person.new('Temperance', 30)

==== Method Call with No Arguments [[method-invocation-parens-no-args]][[method-call-parens-no-args]]

Always omit parentheses for method calls with no arguments.

[source,ruby]

bad

Kernel.exit!() 2.even?() fork() 'test'.upcase()

good

Kernel.exit! 2.even? fork 'test'.upcase

==== Methods That Have "keyword" Status in Ruby [[method-invocation-parens-keyword]][[method-call-parens-keyword]]

Always omit parentheses for methods that have "keyword" status in Ruby.

NOTE: Unfortunately, it's not exactly clear which methods have "keyword" status. There is agreement that declarative methods have "keyword" status. However, there's less agreement on which non-declarative methods, if any, have "keyword" status.

===== Non-Declarative Methods That Have "keyword" Status in Ruby [[method-invocation-parens-non-declarative-keyword]][[method-call-parens-non-declarative-keyword]]

For non-declarative methods with "keyword" status (e.g., various Kernel instance methods), two styles are considered acceptable. By far the most popular style is to omit parentheses. Rationale: The code reads better, and method calls look more like keywords. A less-popular style, but still acceptable, is to include parentheses. Rationale: The methods have ordinary semantics, so why treat them differently, and it's easier to achieve a uniform style by not worrying about which methods have "keyword" status. Whichever one you pick, apply it consistently.

[source,ruby]

good (most popular)

puts temperance.age system 'ls' exit 1

also good (less popular)

puts(temperance.age) system('ls') exit(1)

==== Using super with Arguments [[super-with-args]]

Always use parentheses when calling super with arguments:

[source,ruby]

bad

super name, age

good

super(name, age)

IMPORTANT: When calling super without arguments, super and super() mean different things. Decide what is appropriate for your usage.

=== Too Many Params [[too-many-params]]

Avoid parameter lists longer than three or four parameters.

=== Optional Arguments [[optional-arguments]]

Define optional arguments at the end of the list of arguments. Ruby has some unexpected results when calling methods that have optional arguments at the front of the list.

[source,ruby]

bad

def some_method(a = 1, b = 2, c, d) puts "#{a}, #{b}, #{c}, #{d}" end

some_method('w', 'x') # => '1, 2, w, x' some_method('w', 'x', 'y') # => 'w, 2, x, y' some_method('w', 'x', 'y', 'z') # => 'w, x, y, z'

good

def some_method(c, d, a = 1, b = 2) puts "#{a}, #{b}, #{c}, #{d}" end

some_method('w', 'x') # => '1, 2, w, x' some_method('w', 'x', 'y') # => 'y, 2, w, x' some_method('w', 'x', 'y', 'z') # => 'y, z, w, x'

=== Keyword Arguments Order

Put required keyword arguments before optional keyword arguments. Otherwise, it's much harder to spot optional keyword arguments there, if they're hidden somewhere in the middle.

[source,ruby]

bad

def some_method(foo: false, bar:, baz: 10)

body omitted

end

good

def some_method(bar:, foo: false, baz: 10)

body omitted

end

=== Boolean Keyword Arguments [[boolean-keyword-arguments]]

Use keyword arguments when passing a boolean argument to a method.

[source,ruby]

bad

def some_method(bar = false) puts bar end

bad - common hack before keyword args were introduced

def some_method(options = {}) bar = options.fetch(:bar, false) puts bar end

good

def some_method(bar: false) puts bar end

some_method # => false some_method(bar: true) # => true

=== Keyword Arguments vs Optional Arguments [[keyword-arguments-vs-optional-arguments]]

Prefer keyword arguments over optional arguments.

[source,ruby]

bad

def some_method(a, b = 5, c = 1)

body omitted

end

good

def some_method(a, b: 5, c: 1)

body omitted

end

=== Keyword Arguments vs Option Hashes [[keyword-arguments-vs-option-hashes]]

Use keyword arguments instead of option hashes.

[source,ruby]

bad

def some_method(options = {}) bar = options.fetch(:bar, false) puts bar end

good

def some_method(bar: false) puts bar end

=== Merging Keyword Arguments [[merging-keyword-arguments]]

When passing an existing hash as keyword arguments, add additional arguments directly rather than using merge.

[source,ruby]

bad

some_method(**opts.merge(foo: true))

good

some_method(**opts, foo: true)

=== Arguments Forwarding [[arguments-forwarding]]

Use Ruby 2.7's arguments forwarding.

[source,ruby]

bad

def some_method(*args, &block) other_method(*args, &block) end

bad

def some_method(*args, **kwargs, &block) other_method(*args, **kwargs, &block) end

bad

Please note that it can cause unexpected incompatible behavior

because ... forwards block also.

https://github.com/rubocop/rubocop/issues/7549

def some_method(*args) other_method(*args) end

good

def some_method(...) other_method(...) end

=== Block Forwarding

Use Ruby 3.1's anonymous block forwarding.

In most cases, block argument is given name similar to &block or &proc. Their names have no information and & will be sufficient for syntactic meaning.

[source,ruby]

bad

def some_method(&block) other_method(&block) end

good

def some_method(&) other_method(&) end

=== Private Global Methods [[private-global-methods]]

If you really need "global" methods, add them to Kernel and make them private.

== Classes & Modules

=== Consistent Classes [[consistent-classes]]

Use a consistent structure in your class definitions.

[source,ruby]

class Person

extend/include/prepend go first

extend SomeModule include AnotherModule prepend YetAnotherModule

inner classes

class CustomError < StandardError end

constants are next

SOME_CONSTANT = 20

afterwards we have attribute macros

attr_reader :name

followed by other macros (if any)

validates :name

public class methods are next in line

def self.some_method end

initialization goes between class methods and other instance methods

def initialize end

followed by other public instance methods

def some_method end

protected and private methods are grouped near the end

protected

def some_protected_method end

private

def some_private_method end end

=== Mixin Grouping [[mixin-grouping]]

Split multiple mixins into separate statements.

[source,ruby]

bad

class Person include Foo, Bar end

good

class Person

multiple mixins go in separate statements

include Foo include Bar end

=== Single-line Classes [[single-line-classes]]

Prefer a two-line format for class definitions with no body. It is easiest to read, understand, and modify.

[source,ruby]

bad

FooError = Class.new(StandardError)

okish

class FooError < StandardError; end

ok

class FooError < StandardError end

NOTE: Many editors/tools will fail to understand properly the usage of Class.new. Someone trying to locate the class definition might try a grep "class FooError". A final difference is that the name of your class is not available to the inherited callback of the base class with the Class.new form. In general it's better to stick to the basic two-line style.

=== File Classes [[file-classes]]

Don't nest multi-line classes within classes. Try to have such nested classes each in their own file in a folder named like the containing class.

[source,ruby]

bad

foo.rb

class Foo class Bar # 30 methods inside end

class Car # 20 methods inside end

30 methods inside

end

good

foo.rb

class Foo

30 methods inside

end

foo/bar.rb

class Foo class Bar # 30 methods inside end end

foo/car.rb

class Foo class Car # 20 methods inside end end

=== Namespace Definition [[namespace-definition]]

Define (and reopen) namespaced classes and modules using explicit nesting. Using the scope resolution operator can lead to surprising constant lookups due to Ruby's https://cirw.in/blog/constant-lookup.html[lexical scoping], which depends on the module nesting at the point of definition.

[source,ruby]

module Utilities class Queue end end

bad

class Utilities::Store Module.nesting # => [Utilities::Store]

def initialize # Refers to the top level ::Queue class because Utilities isn't in the # current nesting chain. @queue = Queue.new end end

good

module Utilities class WaitingList Module.nesting # => [Utilities::WaitingList, Utilities]

def initialize
  @queue = Queue.new # Refers to Utilities::Queue
end

end end

=== Modules vs Classes [[modules-vs-classes]]

Prefer modules to classes with only class methods. Classes should be used only when it makes sense to create instances out of them.

[source,ruby]

bad

class SomeClass def self.some_method # body omitted end

def self.some_other_method # body omitted end end

good

module SomeModule module_function

def some_method # body omitted end

def some_other_method # body omitted end end

=== module_function [[module-function]]

Prefer the use of module_function over extend self when you want to turn a module's instance methods into class methods.

[source,ruby]

bad

module Utilities extend self

def parse_something(string) # do stuff here end

def other_utility_method(number, string) # do some more stuff end end

good

module Utilities module_function

def parse_something(string) # do stuff here end

def other_utility_method(number, string) # do some more stuff end end

=== Liskov [[liskov]]

When designing class hierarchies make sure that they conform to the https://en.wikipedia.org/wiki/Liskov_substitution_principle[Liskov Substitution Principle].

=== SOLID design [[solid-design]]

Try to make your classes as https://en.wikipedia.org/wiki/SOLID[SOLID] as possible.

=== Define to_s [[define-to-s]]

Always supply a proper to_s method for classes that represent domain objects.

[source,ruby]

class Person attr_reader :first_name, :last_name

def initialize(first_name, last_name) @first_name = first_name @last_name = last_name end

def to_s "#{first_name} #{last_name}" end end

=== attr Family [[attr_family]]

Use the attr family of functions to define trivial accessors or mutators.

[source,ruby]

bad

class Person def initialize(first_name, last_name) @first_name = first_name @last_name = last_name end

def first_name @first_name end

def last_name @last_name end end

good

class Person attr_reader :first_name, :last_name

def initialize(first_name, last_name) @first_name = first_name @last_name = last_name end end

=== Accessor/Mutator Method Names [[accessor_mutator_method_names]]

For accessors and mutators, avoid prefixing method names with get_ and set_. It is a Ruby convention to use attribute names for accessors (readers) and attr_name= for mutators (writers).

[source,ruby]

bad

class Person def get_name "#{@first_name} #{@last_name}" end

def set_name(name) @first_name, @last_name = name.split(' ') end end

good

class Person def name "#{@first_name} #{@last_name}" end

def name=(name) @first_name, @last_name = name.split(' ') end end

=== attr [[attr]]

Avoid the use of attr. Use attr_reader and attr_accessor instead.

[source,ruby]

bad - creates a single attribute accessor (deprecated in Ruby 1.9)

attr :something, true attr :one, :two, :three # behaves as attr_reader

good

attr_accessor :something attr_reader :one, :two, :three

=== Struct.new [[struct-new]]

Consider using Struct.new, which defines the trivial accessors, constructor and comparison operators for you.

[source,ruby]

good

class Person attr_accessor :first_name, :last_name

def initialize(first_name, last_name) @first_name = first_name @last_name = last_name end end

better

Person = Struct.new(:first_name, :last_name) do end

=== Don't Extend Struct.new [[no-extend-struct-new]]

Don't extend an instance initialized by Struct.new. Extending it introduces a superfluous class level and may also introduce weird errors if the file is required multiple times.

[source,ruby]

bad

class Person < Struct.new(:first_name, :last_name) end

good

Person = Struct.new(:first_name, :last_name)

=== Don't Extend Data.define [[no-extend-data-define]]

Don't extend an instance initialized by Data.define. Extending it introduces a superfluous class level.

[source,ruby]

bad

class Person < Data.define(:first_name, :last_name) end

Person.ancestors

=> [Person, #Class:0x0000000105abed88, Data, Object, (...)]

good

Person = Data.define(:first_name, :last_name)

Person.ancestors

=> [Person, Data, Object, (...)]

---

=== Duck Typing [[duck-typing]]

Prefer https://en.wikipedia.org/wiki/Duck_typing[duck-typing] over inheritance.

[source,ruby]

bad

class Animal

abstract method

def speak end end

extend superclass

class Duck < Animal def speak puts 'Quack! Quack' end end

extend superclass

class Dog < Animal def speak puts 'Bau! Bau!' end end

good

class Duck def speak puts 'Quack! Quack' end end

class Dog def speak puts 'Bau! Bau!' end end

=== No Class Vars [[no-class-vars]]

Avoid the usage of class (@@) variables due to their "nasty" behavior in inheritance.

[source,ruby]

class Parent @@class_var = 'parent'

def self.print_class_var puts @@class_var end end

class Child < Parent @@class_var = 'child' end

Parent.print_class_var # => will print 'child'

As you can see all the classes in a class hierarchy actually share one class variable. Class instance variables should usually be preferred over class variables.

=== Leverage Access Modifiers (e.g. private and protected) [[visibility]]

Assign proper visibility levels to methods (private, protected) in accordance with their intended usage. Don't go off leaving everything public (which is the default).

=== Access Modifiers Indentation [[indent-public-private-protected]]

Indent the public, protected, and private methods as much as the method definitions they apply to. Leave one blank line above the visibility modifier and one blank line below in order to emphasize that it applies to all methods below it.

[source,ruby]

good

class SomeClass def public_method # some code end

private

def private_method # some code end

def another_private_method # some code end end

=== Defining Class Methods [[def-self-class-methods]]

Use def self.method to define class methods. This makes the code easier to refactor since the class name is not repeated.

[source,ruby]

class TestClass

bad

def TestClass.some_method # body omitted end

good

def self.some_other_method # body omitted end

Also possible and convenient when you

have to define many class methods.

class << self def first_method # body omitted end

def second_method_etc
  # body omitted
end

end end

=== Alias Method Lexically [[alias-method-lexically]]

Prefer alias when aliasing methods in lexical class scope as the resolution of self in this context is also lexical, and it communicates clearly to the user that the indirection of your alias will not be altered at runtime or by any subclass unless made explicit.

[source,ruby]

class Westerner def first_name @names.first end

alias given_name first_name end

Since alias, like def, is a keyword, prefer bareword arguments over symbols or strings. In other words, do alias foo bar, not alias :foo :bar.

Also be aware of how Ruby handles aliases and inheritance: an alias references the method that was resolved at the time the alias was defined; it is not dispatched dynamically.

[source,ruby]

class Fugitive < Westerner def first_name 'Nobody' end end

In this example, Fugitive#given_name would still call the original Westerner#first_name method, not Fugitive#first_name. To override the behavior of Fugitive#given_name as well, you'd have to redefine it in the derived class.

[source,ruby]

class Fugitive < Westerner def first_name 'Nobody' end

alias given_name first_name end

=== alias_method [[alias-method]]

Always use alias_method when aliasing methods of modules, classes, or singleton classes at runtime, as the lexical scope of alias leads to unpredictability in these cases.

[source,ruby]

module Mononymous def self.included(other) other.class_eval { alias_method :full_name, :given_name } end end

class Sting < Westerner include Mononymous end

=== Class and self [[class-and-self]]

When class (or module) methods call other such methods, omit the use of a leading self or own name followed by a . when calling other such methods. This is often seen in "service classes" or other similar concepts where a class is treated as though it were a function. This convention tends to reduce repetitive boilerplate in such classes.

[source,ruby]

class TestClass

bad - more work when class renamed/method moved

def self.call(param1, param2) TestClass.new(param1).call(param2) end

bad - more verbose than necessary

def self.call(param1, param2) self.new(param1).call(param2) end

good

def self.call(param1, param2) new(param1).call(param2) end

...other methods...

end

=== Defining Constants within a Block [[no-constant-definition-in-block]]

Do not define constants within a block, since the block's scope does not isolate or namespace the constant in any way.

Define the constant outside of the block instead, or use a variable or method if defining the constant in the outer scope would be problematic.

[source,ruby]

bad - FILES_TO_LINT is now defined globally

task :lint do FILES_TO_LINT = Dir['lib/*.rb']

...

end

good - files_to_lint is only defined inside the block

task :lint do files_to_lint = Dir['lib/*.rb']

...

end

== Classes: Constructors

=== Factory Methods [[factory-methods]]

Consider adding factory methods to provide additional sensible ways to create instances of a particular class.

[source,ruby]

class Person def self.create(options_hash) # body omitted end end

=== Disjunctive Assignment in Constructor [[disjunctive-assignment-in-constructor]]

In constructors, avoid unnecessary disjunctive assignment (||=) of instance variables. Prefer plain assignment. In ruby, instance variables (beginning with an @) are nil until assigned a value, so in most cases the disjunction is unnecessary.

[source,ruby]

bad

def initialize @x ||= 1 end

good

def initialize @x = 1 end

== Comments

[quote, Steve McConnell]

---

Good code is its own best documentation. As you're about to add a comment, ask yourself, "How can I improve the code so that this comment isn't needed?". Improve the code and then document it to make it even clearer.

---

=== No Comments [[no-comments]]

Write self-documenting code and ignore the rest of this section. Seriously!

=== Rationale Comments [[rationale-comments]]

If the how can be made self-documenting, but not the why (e.g. the code works around non-obvious library behavior, or implements an algorithm from an academic paper), add a comment explaining the rationale behind the code.

[source,ruby]

bad

x = BuggyClass.something.dup

def compute_dependency_graph ...30 lines of recursive graph merging... end

good

BuggyClass returns an internal object, so we have to dup it to modify it.

x = BuggyClass.something.dup

This is algorithm 6.4(a) from Worf & Yar's Amazing Graph Algorithms (2243).

def compute_dependency_graph ...30 lines of recursive graph merging... end

=== English Comments [[english-comments]]

Write comments in English.

=== Hash Space [[hash-space]]

Use one space between the leading # character of the comment and the text of the comment.

=== English Syntax [[english-syntax]]

Comments longer than a word are capitalized and use punctuation. Use https://en.wikipedia.org/wiki/Sentence_spacing[one space] after periods.

=== No Superfluous Comments [[no-superfluous-comments]]

Avoid superfluous comments.

[source,ruby]

bad

counter += 1 # Increments counter by one.

=== Comment Upkeep [[comment-upkeep]]

Keep existing comments up-to-date. An outdated comment is worse than no comment at all.

=== Refactor, Don't Comment [[refactor-dont-comment]]

[quote, old programmers maxim, 'https://eloquentruby.com/blog/2011/03/07/good-code-and-good-jokes/[through Russ Olsen]']

---

Good code is like a good joke: it needs no explanation.

---

Avoid writing comments to explain bad code. Refactor the code to make it self-explanatory. ("Do or do not - there is no try." Yoda)

== Comment Annotations

=== Annotations Placement [[annotate-above]]

Annotations should usually be written on the line immediately above the relevant code.

[source,ruby]

bad

def bar baz(:quux) # FIXME: This has crashed occasionally since v3.2.1. end

good

def bar

FIXME: This has crashed occasionally since v3.2.1.

baz(:quux) end

=== Annotations Keyword Format [[annotate-keywords]]

The annotation keyword is followed by a colon and a space, then a note describing the problem.

[source,ruby]

bad

def bar

FIXME This has crashed occasionally since v3.2.1.

baz(:quux) end

good

def bar

FIXME: This has crashed occasionally since v3.2.1.

baz(:quux) end

=== Multi-line Annotations Indentation [[indent-annotations]]

If multiple lines are required to describe the problem, subsequent lines should be indented three spaces after the # (one general plus two for indentation purposes).

[source,ruby]

def bar

FIXME: This has crashed occasionally since v3.2.1. It may

be related to the BarBazUtil upgrade.

baz(:quux) end

=== Inline Annotations [[rare-eol-annotations]]

In cases where the problem is so obvious that any documentation would be redundant, annotations may be left at the end of the offending line with no note. This usage should be the exception and not the rule.

[source,ruby]

def bar sleep 100 # OPTIMIZE end

=== TODO [[todo]]

Use TODO to note missing features or functionality that should be added at a later date.

=== FIXME [[fixme]]

Use FIXME to note broken code that needs to be fixed.

=== OPTIMIZE [[optimize]]

Use OPTIMIZE to note slow or inefficient code that may cause performance problems.

=== HACK [[hack]]

Use HACK to note code smells where questionable coding practices were used and should be refactored away.

=== REVIEW [[review]]

Use REVIEW to note anything that should be looked at to confirm it is working as intended. For example: REVIEW: Are we sure this is how the client does X currently?

=== Document Annotations [[document-annotations]]

Use other custom annotation keywords if it feels appropriate, but be sure to document them in your project's README or similar.

== Magic Comments

=== Magic Comments First [[magic-comments-first]]

Place magic comments above all code and documentation in a file (except shebangs, which are discussed next).

[source,ruby]

bad

Some documentation about Person

frozen_string_literal: true

class Person end

good

frozen_string_literal: true

Some documentation about Person

class Person end

=== Below Shebang [[below-shebang]]

Place magic comments below shebangs when they are present in a file.

[source,ruby]

bad

frozen_string_literal: true

#!/usr/bin/env ruby

App.parse(ARGV)

good

#!/usr/bin/env ruby

frozen_string_literal: true

App.parse(ARGV)

=== One Magic Comment per Line [[one-magic-comment-per-line]]

Use one magic comment per line if you need multiple.

[source,ruby]

bad

-- frozen_string_literal: true; encoding: ascii-8bit --

good

frozen_string_literal: true

encoding: ascii-8bit

---

=== Separate Magic Comments from Code [[separate-magic-comments-from-code]]

Separate magic comments from code and documentation with a blank line.

[source,ruby]

bad

frozen_string_literal: true

Some documentation for Person

class Person

Some code

end

good

frozen_string_literal: true

Some documentation for Person

class Person

Some code

end

== Collections

=== Literal Array and Hash [[literal-array-hash]]

Prefer literal array and hash creation notation (unless you need to pass parameters to their constructors, that is).

[source,ruby]

bad

arr = Array.new hash = Hash.new

good

arr = [] arr = Array.new(10) hash = {} hash = Hash.new(0)

=== %w [[percent-w]]

Prefer %w to the literal array syntax when you need an array of words (non-empty strings without spaces and special characters in them). Apply this rule only to arrays with two or more elements.

[source,ruby]

bad

STATES = ['draft', 'open', 'closed']

good

STATES = %w[draft open closed]

=== %i [[percent-i]]

Prefer %i to the literal array syntax when you need an array of symbols (and you don't need to maintain Ruby 1.9 compatibility). Apply this rule only to arrays with two or more elements.

[source,ruby]

bad

STATES = [:draft, :open, :closed]

good

STATES = %i[draft open closed]

=== No Trailing Array Commas [[no-trailing-array-commas]]

Avoid comma after the last item of an Array or Hash literal, especially when the items are not on separate lines.

[source,ruby]

bad - easier to move/add/remove items, but still not preferred

VALUES = [ 1001, 2020, 3333, ]

bad

VALUES = [1001, 2020, 3333, ]

good

VALUES = [1001, 2020, 3333]

=== No Gappy Arrays [[no-gappy-arrays]]

Avoid the creation of huge gaps in arrays.

[source,ruby]

arr = [] arr[100] = 1 # now you have an array with lots of nils

=== first and last [[first-and-last]]

When accessing the first or last element from an array, prefer first or last over [0] or [-1]. first and last take less effort to understand, especially for a less experienced Ruby programmer or someone from a language with different indexing semantics.

[source,ruby]

arr = [1, 2, 3]

ok

arr[0] # => 1 arr[-1] # => 3

(arguably) better

arr.first # => 1 arr.last # => 3

good - assignments can only be done via []=

arr[0] = 2 arr[-1] = 5

=== Set vs Array [[set-vs-array]]

Use Set instead of Array when dealing with unique elements. Set implements a collection of unordered values with no duplicates. This is a hybrid of Array's intuitive inter-operation facilities and Hash's fast lookup.

=== Symbols as Keys [[symbols-as-keys]]

Prefer symbols instead of strings as hash keys.

[source,ruby]

bad

hash = { 'one' => 1, 'two' => 2, 'three' => 3 }

good

hash = { one: 1, two: 2, three: 3 }

=== No Mutable Keys [[no-mutable-keys]]

Avoid the use of mutable objects as hash keys.

=== Hash Literals [[hash-literals]]

Use the Ruby 1.9 hash literal syntax when your hash keys are symbols.

[source,ruby]

bad

hash = { :one => 1, :two => 2, :three => 3 }

good

hash = { one: 1, two: 2, three: 3 }

=== Hash Literal Values

Use the Ruby 3.1 hash literal value syntax when your hash key and value are the same.

[source,ruby]

bad

hash = { one: one, two: two, three: three }

good

hash = { one:, two:, three: }

=== Hash Literal as Last Array Item [[hash-literal-as-last-array-item]]

Wrap hash literal in braces if it is a last array item.

[source,ruby]

bad

[1, 2, one: 1, two: 2]

good

[1, 2, { one: 1, two: 2 }]

=== No Mixed Hash Syntaxes [[no-mixed-hash-syntaxes]]

Don't mix the Ruby 1.9 hash syntax with hash rockets in the same hash literal. When you've got keys that are not symbols stick to the hash rockets syntax.

[source,ruby]

bad

{ a: 1, 'b' => 2 }

good

{ :a => 1, 'b' => 2 }

=== Avoid Hash[] constructor [[avoid-hash-constructor]]

Hash::[] was a pre-Ruby 2.1 way of constructing hashes from arrays of key-value pairs, or from a flat list of keys and values. It has an obscure semantic and looks cryptic in code. Since Ruby 2.1, Enumerable#to_h can be used to construct a hash from a list of key-value pairs, and it should be preferred. Instead of Hash[] with a list of literal keys and values, just a hash literal should be preferred.

[source,ruby]

bad

Hash[ary] Hash[a, b, c, d]

good

ary.to_h {a => b, c => d}

=== Hash#key? [[hash-key]]

Use Hash#key? instead of Hash#has_key? and Hash#value? instead of Hash#has_value?.

[source,ruby]

bad

hash.has_key?(:test) hash.has_value?(value)

good

hash.key?(:test) hash.value?(value)

=== Hash#each [[hash-each]]

Use Hash#each_key instead of Hash#keys.each and Hash#each_value instead of Hash#values.each.

[source,ruby]

bad

hash.keys.each { |k| p k } hash.values.each { |v| p v } hash.each { |k, _v| p k } hash.each { |_k, v| p v }

good

hash.each_key { |k| p k } hash.each_value { |v| p v }

=== Hash#fetch [[hash-fetch]]

Use Hash#fetch when dealing with hash keys that should be present.

[source,ruby]

heroes = { batman: 'Bruce Wayne', superman: 'Clark Kent' }

bad - if we make a mistake we might not spot it right away

heroes[:batman] # => 'Bruce Wayne' heroes[:supermann] # => nil

good - fetch raises a KeyError making the problem obvious

heroes.fetch(:supermann)

=== Hash#fetch defaults [[hash-fetch-defaults]]

Introduce default values for hash keys via Hash#fetch as opposed to using custom logic.

[source,ruby]

batman = { name: 'Bruce Wayne', is_evil: false }

bad - if we just use || operator with falsey value we won't get the expected result

batman[:is_evil] || true # => true

good - fetch works correctly with falsey values

batman.fetch(:is_evil, true) # => false

=== Use Hash Blocks [[use-hash-blocks]]

Prefer the use of the block instead of the default value in Hash#fetch if the code that has to be evaluated may have side effects or be expensive.

[source,ruby]

batman = { name: 'Bruce Wayne' }

bad - if we use the default value, we eager evaluate it

so it can slow the program down if done multiple times

batman.fetch(:powers, obtain_batman_powers) # obtain_batman_powers is an expensive call

good - blocks are lazy evaluated, so only triggered in case of KeyError exception

batman.fetch(:powers) { obtain_batman_powers }

=== Hash#values_at and Hash#fetch_values [[hash-values-at-and-hash-fetch-values]]

Use Hash#values_at or Hash#fetch_values when you need to retrieve several values consecutively from a hash.

[source,ruby]

bad

email = data['email'] username = data['nickname']

bad

keys = %w[email nickname].freeze email, username = keys.map { |key| data[key] }

good

email, username = data.values_at('email', 'nickname')

also good

email, username = data.fetch_values('email', 'nickname')

=== Hash#transform_keys and Hash#transform_values [[hash-transform-methods]]

Prefer transform_keys or transform_values over each_with_object or map when transforming just the keys or just the values of a hash.

[source,ruby]

bad

{a: 1, b: 2}.each_with_object({}) { |(k, v), h| h[k] = v * v } {a: 1, b: 2}.map { |k, v| [k.to_s, v] }.to_h

good

{a: 1, b: 2}.transform_values { |v| v * v } {a: 1, b: 2}.transform_keys { |k| k.to_s }

=== Ordered Hashes [[ordered-hashes]]

Rely on the fact that as of Ruby 1.9 hashes are ordered.

=== No Modifying Collections [[no-modifying-collections]]

Do not modify a collection while traversing it.

=== Accessing Elements Directly [[accessing-elements-directly]]

When accessing elements of a collection, avoid direct access via [n] by using an alternate form of the reader method if it is supplied. This guards you from calling [] on nil.

[source,ruby]

bad

Regexp.last_match[1]

good

Regexp.last_match(1)

=== Provide Alternate Accessor to Collections [[provide-alternate-accessor-to-collections]]

When providing an accessor for a collection, provide an alternate form to save users from checking for nil before accessing an element in the collection.

[source,ruby]

bad

def awesome_things @awesome_things end

good

def awesome_things(index = nil) if index && @awesome_things @awesome_things[index] else @awesome_things end end

=== map/find/select/reduce/include?/size [[map-find-select-reduce-include-size]]

Prefer map over collect, find over detect, select over find_all, reduce over inject, include? over member? and size over length. This is not a hard requirement; if the use of the alias enhances readability, it's ok to use it. The rhyming methods are inherited from Smalltalk and are not common in other programming languages. The reason the use of select is encouraged over find_all is that it goes together nicely with reject and its name is pretty self-explanatory.

=== count vs size [[count-vs-size]]

Don't use count as a substitute for size. For Enumerable objects other than Array it will iterate the entire collection in order to determine its size.

[source,ruby]

bad

some_hash.count

good

some_hash.size

=== flat_map [[flat-map]]

Use flat_map instead of map + flatten. This does not apply for arrays with a depth greater than 2, i.e. if users.first.songs == ['a', ['b','c']], then use map + flatten rather than flat_map. flat_map flattens the array by 1, whereas flatten flattens it all the way.

[source,ruby]

bad

all_songs = users.map(&:songs).flatten.uniq

good

all_songs = users.flat_map(&:songs).uniq

=== reverse_each [[reverse-each]]

Prefer reverse_each to reverse.each because some classes that include Enumerable will provide an efficient implementation. Even in the worst case where a class does not provide a specialized implementation, the general implementation inherited from Enumerable will be at least as efficient as using reverse.each.

[source,ruby]

bad

array.reverse.each { ... }

good

array.reverse_each { ... }

=== Object#yield_self vs Object#then [[object-yield-self-vs-object-then]]

The method Object#then is preferred over Object#yield_self, since the name then states the intention, not the behavior. This makes the resulting code easier to read.

[source,ruby]

bad

obj.yield_self { |x| x.do_something }

good

obj.then { |x| x.do_something }

NOTE: You can read more about the rationale behind this guideline https://bugs.ruby-lang.org/issues/14594[here].

=== Slicing with Ranges

Slicing arrays with ranges to extract some of their elements (e.g ary[2..5]) is a popular technique. Below you'll find a few small considerations to keep in mind when using it.

• [0..-1] in ary[0..-1] is redundant and simply synonymous with ary.

[source,ruby]

bad - you're selecting all the elements of the array

ary[0..-1] ary[0..nil] ary[0...nil]

good

ary

• Ruby 2.6 introduced endless ranges, which provide an easier way to describe a slice going all the way to the end of an array.

[source,ruby]

bad - hard to process mentally

ary[1..-1] ary[1..nil]

good - easier to read and more concise

ary[1..]

• Ruby 2.7 introduced beginless ranges, which are also handy in slicing. However, unlike the somewhat obscure -1 in ary[1..-1], the 0 in ary[0..42] is clear as a starting point. In fact, changing it to ary[..42] could potentially make it less readable. Therefore, using code like ary[0..42] is fine. On the other hand, ary[nil..42] should be replaced with ary[..42] or arr[0..42].

[source,ruby]

bad - hard to process mentally

ary[nil..42]

good - easier to read

ary[..42] ary[0..42]

== Numbers

=== Underscores in Numerics [[underscores-in-numerics]]

Add underscores to large numeric literals to improve their readability.

[source,ruby]

bad - how many 0s are there?

num = 1000000

good - much easier to parse for the human brain

num = 1_000_000

=== Numeric Literal Prefixes [[numeric-literal-prefixes]]

Prefer lowercase letters for numeric literal prefixes. 0o for octal, 0x for hexadecimal and 0b for binary. Do not use 0d prefix for decimal literals.

[source,ruby]

bad

num = 01234 num = 0O1234 num = 0X12AB num = 0B10101 num = 0D1234 num = 0d1234

good - easier to separate digits from the prefix

num = 0o1234 num = 0x12AB num = 0b10101 num = 1234

=== Integer Type Checking [[integer-type-checking]]

Use Integer to check the type of an integer number. Since Fixnum is platform-dependent, checking against it will return different results on 32-bit and 64-bit machines.

[source,ruby]

timestamp = Time.now.to_i

bad

timestamp.is_a?(Fixnum) timestamp.is_a?(Bignum)

good

timestamp.is_a?(Integer)

=== Random Numbers [[random-numbers]]

Prefer to use ranges when generating random numbers instead of integers with offsets, since it clearly states your intentions. Imagine simulating a roll of a dice:

[source,ruby]

bad

rand(6) + 1

good

rand(1..6)

=== Float Division [[float-division]]

When performing float-division on two integers, either use fdiv or convert one-side integer to float.

[source,ruby]

bad

a.to_f / b.to_f

good

a.to_f / b a / b.to_f a.fdiv(b)

=== Float Comparison [[float-comparison]]

Avoid (in)equality comparisons of floats as they are unreliable.

Floating point values are inherently inaccurate, and comparing them for exact equality is almost never the desired semantics. Comparison via the ==/!= operators checks floating-point value representation to be exactly the same, which is very unlikely if you perform any arithmetic operations involving precision loss.

[source,ruby]

bad

x == 0.1 x != 0.1

good - using BigDecimal

x.to_d == 0.1.to_d

good - not an actual float comparison

x == Float::INFINITY

good

(x - 0.1).abs < Float::EPSILON

good

tolerance = 0.0001 (x - 0.1).abs < tolerance

Or some other epsilon based type of comparison:

https://www.embeddeduse.com/2019/08/26/qt-compare-two-floats/

---

=== Exponential Notation [[exponential-notation]]

When using exponential notation for numbers, prefer using the normalized scientific notation, which uses a mantissa between 1 (inclusive) and 10 (exclusive). Omit the exponent altogether if it is zero.

The goal is to avoid confusion between powers of ten and exponential notation, as one quickly reading 10e7 could think it's 10 to the power of 7 (one then 7 zeroes) when it's actually 10 to the power of 8 (one then 8 zeroes). If you want 10 to the power of 7, you should do 1e7.

|=== | power notation | exponential notation | output

| 10 ** 7 | 1e7 | 10000000 | 10 ** 6 | 1e6 | 1000000 | 10 ** 7 | 10e6 | 10000000 |===

One could favor the alternative engineering notation, in which the exponent must always be a multiple of 3 for easy conversion to the thousand / million / ... system.

[source,ruby]

bad

10e6 0.3e4 11.7e5 3.14e0

good

1e7 3e3 1.17e6 3.14

Alternative : engineering notation:

[source,ruby]

bad

3.2e7 0.1e5 12e4

good

1e6 17e6 0.98e9

== Strings

=== String Interpolation [[string-interpolation]]

Prefer string interpolation and string formatting to string concatenation:

[source,ruby]

bad

email_with_name = user.name + ' <' + user.email + '>'

good

email_with_name = "#{user.name} <#{user.email}>"

good

email_with_name = format('%s <%s>', user.name, user.email)

=== Consistent String Literals [[consistent-string-literals]]

Adopt a consistent string literal quoting style. There are two popular styles in the Ruby community, both of which are considered good - single quotes by default and double quotes by default.

NOTE: The string literals in this guide are using single quotes by default.

==== Single Quote [[consistent-string-literals-single-quote]]

Prefer single-quoted strings when you don't need string interpolation or special symbols such as \t, \n, ', etc.

[source,ruby]

bad

name = "Bozhidar"

name = 'De'Andre'

good

name = 'Bozhidar'

name = "De'Andre"

==== Double Quote [[consistent-string-literals-double-quote]]

Prefer double-quotes unless your string literal contains " or escape characters you want to suppress.

[source,ruby]

bad

name = 'Bozhidar'

sarcasm = "I "like" it."

good

name = "Bozhidar"

sarcasm = 'I "like" it.'

=== No Character Literals [[no-character-literals]]

Don't use the character literal syntax ?x. Since Ruby 1.9 it's basically redundant - ?x would be interpreted as 'x' (a string with a single character in it).

[source,ruby]

bad

char = ?c

good

char = 'c'

=== Curlies Interpolate [[curlies-interpolate]]

Don't leave out {} around instance and global variables being interpolated into a string.

[source,ruby]

class Person attr_reader :first_name, :last_name

def initialize(first_name, last_name) @first_name = first_name @last_name = last_name end

bad - valid, but awkward

def to_s "#@first_name #@last_name" end

good

def to_s "#{@first_name} #{@last_name}" end end

$global = 0

bad

puts "$global = #$global"

good

puts "$global = #{$global}"

=== No to_s [[no-to-s]]

Don't use Object#to_s on interpolated objects. It's called on them automatically.

[source,ruby]

bad

message = "This is the #{result.to_s}."

good

message = "This is the #{result}."

=== String Concatenation [[concat-strings]]

Avoid using pass:[String#+] when you need to construct large data chunks. Instead, use String#<<. Concatenation mutates the string instance in-place and is always faster than pass:[String#+], which creates a bunch of new string objects.

[source,ruby]

bad

html = '' html += 'Page title'

paragraphs.each do |paragraph| html += "#{paragraph}" end

good and also fast

html = '' html << 'Page title'

paragraphs.each do |paragraph| html << "#{paragraph}" end

=== Don't Abuse gsub [[dont-abuse-gsub]]

Don't use String#gsub in scenarios in which you can use a faster and more specialized alternative.

[source,ruby]

url = 'http://example.com' str = 'lisp-case-rules'

bad

url.gsub('http://', 'https://') str.gsub('-', '_')

good

url.sub('http://', 'https://') str.tr('-', '_')

=== String#chars [[string-chars]]

Prefer the use of String#chars over String#split with empty string or regexp literal argument.

NOTE: These cases have the same behavior since Ruby 2.0.

[source,ruby]

bad

string.split(//) string.split('')

good

string.chars

=== sprintf [[sprintf]]

Prefer the use of sprintf and its alias format over the fairly cryptic String#% method.

[source,ruby]

bad

'%d %d' % [20, 10]

=> '20 10'

good

sprintf('%d %d', 20, 10)

=> '20 10'

good

sprintf('%d %d', first: 20, second: 10)

=> '20 10'

format('%d %d', 20, 10)

=> '20 10'

good

format('%d %d', first: 20, second: 10)

=> '20 10'

---

=== Named Format Tokens [[named-format-tokens]]

When using named format string tokens, favor %<name>s over %{name} because it encodes information about the type of the value.

[source,ruby]

bad

format('Hello, %{name}', name: 'John')

good

format('Hello, %s', name: 'John')

=== Long Strings [[heredoc-long-strings]]

Break long strings into multiple lines but don't concatenate them with +. If you want to add newlines, use heredoc. Otherwise use \:

[source,ruby]

bad

"Lorem Ipsum is simply dummy text of the printing and typesetting industry. " + "Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, " + "when an unknown printer took a galley of type and scrambled it to make a type specimen book."

good

<<~LOREM Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. LOREM

good

"Lorem Ipsum is simply dummy text of the printing and typesetting industry. "
"Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, "
"when an unknown printer took a galley of type and scrambled it to make a type specimen book."

== Heredocs

=== Squiggly Heredocs [[squiggly-heredocs]]

Use Ruby 2.3's squiggly heredocs for nicely indented multi-line strings.

[source,ruby]

bad - using Powerpack String#strip_margin

code = <<-RUBY.strip_margin('|') |def test | some_method | other_method |end RUBY

also bad

code = <<-RUBY def test some_method other_method end RUBY

good

code = <<~RUBY def test some_method other_method end RUBY

=== Heredoc Delimiters [[heredoc-delimiters]]

Use descriptive delimiters for heredocs. Delimiters add valuable information about the heredoc content, and as an added bonus some editors can highlight code within heredocs if the correct delimiter is used.

[source,ruby]

bad

code = <<~END def foo bar end END

good

code = <<~RUBY def foo bar end RUBY

good

code = <<~SUMMARY An imposing black structure provides a connection between the past and the future in this enigmatic adaptation of a short story by revered sci-fi author Arthur C. Clarke. SUMMARY

=== Heredoc Method Calls [[heredoc-method-calls]]

Place method calls with heredoc receivers on the first line of the heredoc definition. The bad form has significant potential for error if a new line is added or removed.

[source,ruby]

bad

query = <<~SQL select foo from bar SQL .strip_indent

good

query = <<~SQL.strip_indent select foo from bar SQL

=== Heredoc Argument Closing Parentheses [[heredoc-argument-closing-parentheses]]

Place the closing parenthesis for method calls with heredoc arguments on the first line of the heredoc definition. The bad form has potential for error if the new line before the closing parenthesis is removed.

[source,ruby]

bad

foo(<<~SQL select foo from bar SQL )

good

foo(<<~SQL) select foo from bar SQL

== Date & Time

=== Time.now [[time-now]]

Prefer Time.now over Time.new when retrieving the current system time.

=== No DateTime [[no-datetime]]

Don't use DateTime unless you need to account for historical calendar reform - and if you do, explicitly specify the start argument to clearly state your intentions.

[source,ruby]

bad - uses DateTime for current time

DateTime.now

good - uses Time for current time

Time.now

bad - uses DateTime for modern date

DateTime.iso8601('2016-06-29')

good - uses Date for modern date

Date.iso8601('2016-06-29')

good - uses DateTime with start argument for historical date

DateTime.iso8601('1751-04-23', Date::ENGLAND)

== Regular Expressions

[quote, Jamie Zawinski]

---

Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems.

---

=== Plain Text Search [[no-regexp-for-plaintext]]

Don't use regular expressions if you just need plain text search in string.

[source,ruby]

foo = 'I am an example string'

bad - using a regular expression is an overkill here

foo =~ /example/

good

foo['example']

=== Using Regular Expressions as String Indexes [[regexp-string-index]]

For simple constructions you can use regexp directly through string index.

[source,ruby]

match = string[/regexp/] # get content of matched regexp first_group = string[/text(grp)/, 1] # get content of captured group string[/text (grp)/, 1] = 'replace' # string => 'text replace'

=== Prefer Non-capturing Groups [[non-capturing-regexp]]

Use non-capturing groups when you don't use the captured result.

[source,ruby]

bad

/(first|second)/

good

/(?:first|second)/

=== Do not mix named and numbered captures [[do-not-mix-named-and-numbered-captures]]

Do not mix named captures and numbered captures in a Regexp literal. Because numbered capture is ignored if they're mixed.

[source,ruby]

bad - There is no way to access (BAR) capturing.

m = /(?FOO)(BAR)/.match('FOOBAR') p m[:foo] # => "FOO" p m[1] # => "FOO" p m[2] # => nil - not "BAR"

good - Both captures are accessible with names.

m = /(?FOO)(?BAR)/.match('FOOBAR') p m[:foo] # => "FOO" p m[:bar] # => "BAR"

good - (?:BAR) is non-capturing grouping.

m = /(?FOO)(?:BAR)/.match('FOOBAR') p m[:foo] # => "FOO"

good - Both captures are accessible with numbers.

m = /(FOO)(BAR)/.match('FOOBAR') p m[1] # => "FOO" p m[2] # => "BAR"

=== Refer named regexp captures by name [[refer-named-regexp-captures-by-name]]

Prefer using names to refer named regexp captures instead of numbers.

[source,ruby]

bad

m = /(?FOO)(?BAR)/.match('FOOBAR') p m[1] # => "FOO" p m[2] # => "BAR"

good

m = /(?FOO)(?BAR)/.match('FOOBAR') p m[:foo] # => "FOO" p m[:bar] # => "BAR"

=== Avoid Perl-style Last Regular Expression Group Matchers [[no-perl-regexp-last-matchers]]

Don't use the cryptic Perl-legacy variables denoting last regexp group matches ($1, $2, etc). Use Regexp.last_match(n) instead.

[source,ruby]

/(regexp)/ =~ string ...

bad

process $1

good

process Regexp.last_match(1)

=== Avoid Numbered Groups [[no-numbered-regexes]]

Avoid using numbered groups as it can be hard to track what they contain. Named groups can be used instead.

[source,ruby]

bad

/(regexp)/ =~ string

some code

process Regexp.last_match(1)

good

/(?<meaningful_var>regexp)/ =~ string

some code

process meaningful_var

=== Limit Escapes [[limit-escapes]]

Character classes have only a few special characters you should care about: ^, -, \, ], so don't escape . or brackets in [].

=== Caret and Dollar Regexp [[caret-and-dollar-regexp]]

Be careful with ^ and $ as they match start/end of line, not string endings. If you want to match the whole string use: \A and \z (not to be confused with \Z which is the equivalent of /\n?\z/).

[source,ruby]

string = "some injection\nusername" string[/^username$/] # matches string[/\Ausername\z/] # doesn't match

=== Multi-line Regular Expressions [[multi-line-regexes]]

Use x (free-spacing) modifier for multi-line regexps.

NOTE: That's known as https://www.regular-expressions.info/freespacing.html[free-spacing mode]. In this mode leading and trailing whitespace is ignored.

[source,ruby]

bad

regex = /start
\s
(group)
(?:alt1|alt2)
end/

good

regexp = / start \s (group) (?:alt1|alt2) end /x

=== Comment Complex Regular Expressions [[comment-regexes]]

Use x modifier for complex regexps. This makes them more readable and you can add some useful comments.

[source,ruby]

regexp = / start # some text \s # white space char (group) # first group (?:alt1|alt2) # some alternation end /x

=== Use gsub with a Block or a Hash for Complex Replacements [[gsub-blocks]]

For complex replacements sub/gsub can be used with a block or a hash.

[source,ruby]

words = 'foo bar' words.sub(/f/, 'f' => 'F') # => 'Foo bar' words.gsub(/\w+/) { |word| word.capitalize } # => 'Foo Bar'

== Percent Literals

=== %q shorthand [[percent-q-shorthand]]

Use %() (it's a shorthand for %Q) for single-line strings which require both interpolation and embedded double-quotes. For multi-line strings, prefer heredocs.

[source,ruby]

bad (no interpolation needed)

%(Some text)

should be 'Some text'

bad (no double-quotes)

%(This is #{quality} style)

should be "This is #{quality} style"

bad (multiple lines)

%(\n#{exclamation}\n)

should be a heredoc.

good (requires interpolation, has quotes, single line)

%(#{name})

=== %q [[percent-q]]

Avoid %() or the equivalent %q() unless you have a string with both ' and " in it. Regular string literals are more readable and should be preferred unless a lot of characters would have to be escaped in them.

[source,ruby]

bad

name = %q(Bruce Wayne) time = %q(8 o'clock) question = %q("What did you say?")

good

name = 'Bruce Wayne' time = "8 o'clock" question = '"What did you say?"' quote = %q("What did you say?")

=== %r [[percent-r]]

Use %r only for regular expressions matching at least one / character.

[source,ruby]

bad

%r{\s+}

good

%r{^/(.)$} %r{^/blog/2011/(.)$}

=== %x [[percent-x]]

Avoid the use of %x unless you're going to execute a command with backquotes in it (which is rather unlikely).

[source,ruby]

bad

date = %x(date)

good

date = date echo = %x(echo date)

=== %s [[percent-s]]

Avoid the use of %s. It seems that the community has decided :"some string" is the preferred way to create a symbol with spaces in it.

=== Percent Literal Braces [[percent-literal-braces]]

Use the braces that are the most appropriate for the various kinds of percent literals.

• () for string literals (%q, %Q).
• [] for array literals (%w, %i, %W, %I) as it is aligned with the standard array literals.
• {} for regexp literals (%r) since parentheses often appear inside regular expressions. That's why a less common character with { is usually the best delimiter for %r literals.
• () for all other literals (e.g. %s, %x)

[source,ruby]

bad

%q{"Test's king!", John said.}

good

%q("Test's king!", John said.)

bad

%w(one two three) %i(one two three)

good

%w[one two three] %i[one two three]

bad

%r((\w+)-(\d+)) %r{\w{1,2}\d{2,5}}

good

%r{(\w+)-(\d+)} %r|\w{1,2}\d{2,5}|

== Metaprogramming

=== No Needless Metaprogramming [[no-needless-metaprogramming]]

Avoid needless metaprogramming.

=== No Monkey Patching [[no-monkey-patching]]

Do not mess around in core classes when writing libraries (do not monkey-patch them).

=== Block class_eval [[block-class-eval]]

The block form of class_eval is preferable to the string-interpolated form.

==== Supply Location [[class-eval-supply-location]]

When you use the string-interpolated form, always supply +__FILE__+ and +__LINE__+, so that your backtraces make sense:

[source,ruby]

class_eval 'def use_relative_model_naming?; true; end', FILE, LINE

==== define_method [[class-eval-define_method]]

define_method is preferable to class_eval { def ... }

=== eval Comment Docs [[eval-comment-docs]]

When using class_eval (or other eval) with string interpolation, add a comment block showing its appearance if interpolated (a practice used in Rails code):

[source,ruby]

from activesupport/lib/active_support/core_ext/string/output_safety.rb

UNSAFE_STRING_METHODS.each do |unsafe_method| if 'String'.respond_to?(unsafe_method) class_eval <<-EOT, FILE, LINE + 1 def #{unsafe_method}(*params, &block) # def capitalize(*params, &block) to_str.#{unsafe_method}(*params, &block) # to_str.capitalize(*params, &block) end # end

  def #{unsafe_method}!(*params)              # def capitalize!(*params)
    @dirty = true                             #   @dirty = true
    super                                     #   super
  end                                         # end
EOT

end end

=== No method_missing [[no-method-missing]]

Avoid using method_missing for metaprogramming because backtraces become messy, the behavior is not listed in #methods, and misspelled method calls might silently work, e.g. nukes.luanch_state = false. Consider using delegation, proxy, or define_method instead. If you must use method_missing:

• Be sure to https://blog.marc-andre.ca/2010/11/15/methodmissing-politely/[also define respond_to_missing?]
• Only catch methods with a well-defined prefix, such as find_by_*--make your code as assertive as possible.
• Call super at the end of your statement
• Delegate to assertive, non-magical methods:

[source,ruby]

bad

def method_missing(meth, params, &block) if /^find_by_(?.)/ =~ meth # ... lots of code to do a find_by else super end end

good

def method_missing(meth, params, &block) if /^find_by_(?.)/ =~ meth find_by(prop, *params, &block) else super end end

best of all, though, would to define_method as each findable attribute is declared

---

=== Prefer public_send [[prefer-public-send]]

Prefer public_send over send so as not to circumvent private/protected visibility.

[source,ruby]

We have an ActiveModel Organization that includes concern Activatable

module Activatable extend ActiveSupport::Concern

included do before_create :create_token end

private

def reset_token # some code end

def create_token # some code end

def activate! # some code end end

class Organization < ActiveRecord::Base include Activatable end

linux_organization = Organization.find(...)

bad - violates privacy

linux_organization.send(:reset_token)

good - should throw an exception

linux_organization.public_send(:reset_token)

=== Prefer +__send__+ [[prefer-send]]

Prefer +__send__+ over send, as send may overlap with existing methods.

[source,ruby]

require 'socket'

u1 = UDPSocket.new u1.bind('127.0.0.1', 4913) u2 = UDPSocket.new u2.connect('127.0.0.1', 4913)

bad - Won't send a message to the receiver object. Instead it will send a message via UDP socket.

u2.send :sleep, 0

good - Will actually send a message to the receiver object.

u2.send ...

== API Documentation [[api-documentation]]

=== YARD

Use https://yardoc.org/[YARD] and its conventions for API documentation.

=== RD (Block) Comments [[no-block-comments]]

Don't use block comments. They cannot be preceded by whitespace and are not as easy to spot as regular comments.

[source,ruby]

bad

=begin comment line another comment line =end

good

comment line

another comment line

---

.From Perl's POD to RD

---

This is not really a block comment syntax, but more of an attempt to emulate Perl's https://perldoc.perl.org/perlpod.html[POD] documentation system.

There's an https://github.com/uwabami/rdtool[rdtool] for Ruby that's pretty similar to POD. Basically rdtool scans a file for =begin and =end pairs, and extracts the text between them all. This text is assumed to be documentation in https://github.com/uwabami/rdtool/blob/master/doc/rd-draft.rd[RD format]. You can read more about it https://ruby-doc.com/docs/ProgrammingRuby/html/rdtool.html[here].

RD predated the rise of RDoc and YARD and was effectively obsoleted by them.footnote:[According to this https://en.wikipedia.org/wiki/Ruby_Document_format[Wikipedia article] the format used to be popular until the early 2000s when it was superseded by RDoc.]

---

== Gemfile and Gemspec

=== No RUBY_VERSION in the gemspec [[no-ruby-version-in-the-gemspec]]

The gemspec should not contain RUBY_VERSION as a condition to switch dependencies. RUBY_VERSION is determined by rake release, so users may end up with wrong dependency.

[source,ruby]

bad

Gem::Specification.new do |s| if RUBY_VERSION >= '2.5' s.add_dependency 'gem_a' else s.add_dependency 'gem_b' end end

Fix by either:

• Post-install messages.
• Add both gems as dependency (if permissible).
• If development dependencies, move to Gemfile.

=== add_dependency vs add_runtime_dependency [[add_dependency_vs_add_runtime_dependency]]

Prefer add_dependency over add_runtime_dependency because add_dependency is considered soft-deprecated and the Bundler team recommends add_dependency.

[source,ruby]

bad

Gem::Specification.new do |s| s.add_runtime_dependency 'gem_a' end

good

Gem::Specification.new do |s| s.add_dependency 'gem_a' end

See https://github.com/rubygems/rubygems/issues/7799#issuecomment-2192720316 for details.

== Misc

=== No Flip-flops [[no-flip-flops]]

Avoid the use of https://en.wikipedia.org/wiki/Flip-flop_(programming)[flip-flop operators].

=== No non-nil Checks [[no-non-nil-checks]]

Don't do explicit non-nil checks unless you're dealing with boolean values.

[source,ruby]

bad

do_something if !something.nil? do_something if something != nil

good

do_something if something

good - dealing with a boolean

def value_set? !@some_boolean.nil? end

=== Global Input/Output Streams [[global-stdout]]

Use $stdout/$stderr/$stdin instead of STDOUT/STDERR/STDIN. STDOUT/STDERR/STDIN are constants, and while you can actually reassign (possibly to redirect some stream) constants in Ruby, you'll get an interpreter warning if you do so.

[source,ruby]

bad

STDOUT.puts('hello')

hash = { out: STDOUT, key: value }

def m(out = STDOUT) out.puts('hello') end

good

$stdout.puts('hello')

hash = { out: $stdout, key: value }

def m(out = $stdout) out.puts('hello') end

NOTE: The only valid use-case for the stream constants is obtaining references to the original streams (assuming you've redirected some of the global vars).

=== Warn [[warn]]

Use warn instead of $stderr.puts. Apart from being more concise and clear, warn allows you to suppress warnings if you need to (by setting the warn level to 0 via -W0).

[source,ruby]

bad

$stderr.puts 'This is a warning!'

good

warn 'This is a warning!'

=== Array#join [[array-join]]

Prefer the use of Array#join over the fairly cryptic Array#* with a string argument.

[source,ruby]

bad

%w[one two three] * ', '

=> 'one, two, three'

good

%w[one two three].join(', ')

=> 'one, two, three'

---

=== Array Coercion [[array-coercion]]

Use Array() instead of explicit Array check or [*var], when dealing with a variable you want to treat as an Array, but you're not certain it's an array.

[source,ruby]

bad

paths = [paths] unless paths.is_a?(Array) paths.each { |path| do_something(path) }

bad (always creates a new Array instance)

[*paths].each { |path| do_something(path) }

good (and a bit more readable)

Array(paths).each { |path| do_something(path) }

=== Ranges or between [[ranges-or-between]]

Use ranges or Comparable#between? instead of complex comparison logic when possible.

[source,ruby]

bad

do_something if x >= 1000 && x <= 2000

good

do_something if (1000..2000).include?(x)

good

do_something if x.between?(1000, 2000)

=== Predicate Methods [[predicate-methods]]

Prefer the use of predicate methods to explicit comparisons with ==. Numeric comparisons are OK.

[source,ruby]

bad

if x % 2 == 0 end

if x % 2 == 1 end

if x == nil end

good

if x.even? end

if x.odd? end

if x.nil? end

if x.zero? end

if x == 0 end

=== Bitwise Predicate Methods [[bitwise-predicate-methods]]

Prefer bitwise predicate methods over direct comparison operations.

[source,ruby]

bad - checks any set bits

(variable & flags).positive?

good

variable.anybits?(flags)

bad - checks all set bits

(variable & flags) == flags

good

variable.allbits?(flags)

bad - checks no set bits

(variable & flags).zero? (variable & flags) == 0

good

variable.nobits?(flags)

=== No Cryptic Perlisms [[no-cryptic-perlisms]]

Avoid using Perl-style special variables (like $:, $;, etc). They are quite cryptic and their use in anything but one-liner scripts is discouraged.

[source,ruby]

bad

$:.unshift File.dirname(FILE)

good

$LOAD_PATH.unshift File.dirname(FILE)

Use the human-friendly aliases provided by the English library if required.

[source,ruby]

bad

print $', $$

good

require 'English' print $POSTMATCH, $PID

=== Use require_relative whenever possible

For all your internal dependencies, you should use require_relative. Use of require should be reserved for external dependencies

[source,ruby]

bad

require 'set' require 'my_gem/spec/helper' require 'my_gem/lib/something'

good

require 'set' require_relative 'helper' require_relative '../lib/something'

This way is more expressive (making clear which dependency is internal or not) and more efficient (as require_relative doesn't have to try all of $LOAD_PATH contrary to require).

=== Always Warn [[always-warn]]

Write ruby -w safe code.

=== No Optional Hash Params [[no-optional-hash-params]]

Avoid hashes as optional parameters. Does the method do too much? (Object initializers are exceptions for this rule).

=== Instance Vars [[instance-vars]]

Use module instance variables instead of global variables.

[source,ruby]

bad

$foo_bar = 1

good

module Foo class << self attr_accessor :bar end end

Foo.bar = 1

=== OptionParser [[optionparser]]

Use OptionParser for parsing complex command line options and ruby -s for trivial command line options.

=== No Param Mutations [[no-param-mutations]]

Do not mutate parameters unless that is the purpose of the method.

=== Three is the Number Thou Shalt Count [[three-is-the-number-thou-shalt-count]]

Avoid more than three levels of block nesting.

=== Functional Code [[functional-code]]

Code in a functional way, avoiding mutation when that makes sense.

[source,ruby]

a = []; [1, 2, 3].each { |i| a << i * 2 } # bad a = [1, 2, 3].map { |i| i * 2 } # good

a = {}; [1, 2, 3].each { |i| a[i] = i * 17 } # bad a = [1, 2, 3].reduce({}) { |h, i| h[i] = i * 17; h } # good a = [1, 2, 3].each_with_object({}) { |i, h| h[i] = i * 17 } # good

=== No explicit .rb to require [[no-explicit-rb-to-require]]

Omit the .rb extension for filename passed to require and require_relative.

NOTE: If the extension is omitted, Ruby tries adding '.rb', '.so', and so on to the name until found. If the file named cannot be found, a LoadError will be raised. There is an edge case where foo.so file is loaded instead of a LoadError if foo.so file exists when require 'foo.rb' will be changed to require 'foo', but that seems harmless.

[source,ruby]

bad

require 'foo.rb' require_relative '../foo.rb'

good

require 'foo' require 'foo.so' require_relative '../foo' require_relative '../foo.so'

=== Avoid tap

The method tap can be helpful for debugging purposes but should not be left in production code. [source,ruby]

bad

Config.new(hash, path).tap do |config| config.check if check end

good

config = Config.new(hash, path) config.check if check config

This is simpler and more efficient.

== Tools

Here are some tools to help you automatically check Ruby code against this guide.

=== RuboCop

https://github.com/rubocop/rubocop[RuboCop] is a Ruby static code analyzer and formatter, based on this style guide. RuboCop already covers a significant portion of the guide and has https://docs.rubocop.org/rubocop/integration_with_other_tools.html[plugins] for most popular Ruby editors and IDEs.

TIP: RuboCop's cops (code checks) have links to the guidelines that they are based on, as part of their metadata.

=== RubyMine

https://www.jetbrains.com/ruby/[RubyMine]'s code inspections are https://confluence.jetbrains.com/display/RUBYDEV/RubyMine+Inspections[partially based] on this guide.

== History

This guide started its life in 2011 as an internal company Ruby coding guidelines (written by https://github.com/bbatsov[Bozhidar Batsov]). Bozhidar had always been bothered as a Ruby developer about one thing - Python developers had a great programming style reference (https://www.python.org/dev/peps/pep-0008/[PEP-8]) and Rubyists never got an official guide, documenting Ruby coding style and best practices. Bozhidar firmly believed that style matters. He also believed that a great hacker community, such as Ruby has, should be quite capable of producing this coveted document. The rest is history...

At some point Bozhidar decided that the work he was doing might be interesting to members of the Ruby community in general and that the world had little need for another internal company guideline. But the world could certainly benefit from a community-driven and community-sanctioned set of practices, idioms and style prescriptions for Ruby programming.

Bozhidar served as the guide's only editor for a few years, before a team of editors was formed once the project transitioned to RuboCop HQ.

Since the inception of the guide we've received a lot of feedback from members of the exceptional Ruby community around the world. Thanks for all the suggestions and the support! Together we can make a resource beneficial to each and every Ruby developer out there.

== Sources of Inspiration

Many people, books, presentations, articles and other style guides influenced the community Ruby style guide. Here are some of them:

• https://en.wikipedia.org/wiki/The_Elements_of_Style["The Elements of Style"]
• https://en.wikipedia.org/wiki/The_Elements_of_Programming_Style["The Elements of Programming Style"]
• https://www.python.org/dev/peps/pep-0008/[The Python Style Guide (PEP-8)]
• https://pragprog.com/book/ruby4/programming-ruby-1-9-2-0["Programming Ruby"]
• https://www.amazon.com/Ruby-Programming-Language-David-Flanagan/dp/0596516177["The Ruby Programming Language"]

== Contributing

The guide is still a work in progress - some guidelines are lacking examples, some guidelines don't have examples that illustrate them clearly enough. Improving such guidelines is a great (and simple way) to help the Ruby community!

In due time these issues will (hopefully) be addressed - just keep them in mind for now.

Nothing written in this guide is set in stone. It's our desire to work together with everyone interested in Ruby coding style, so that we could ultimately create a resource that will be beneficial to the entire Ruby community.

Feel free to open tickets or send pull requests with improvements. Thanks in advance for your help!

You can also support the project (and RuboCop) with financial contributions via one of the following platforms:

• https://github.com/sponsors/bbatsov[GitHub Sponsors]
• https://ko-fi.com/bbatsov[ko-fi]
• https://www.patreon.com/bbatsov[Patreon]
• https://www.paypal.me/bbatsov[PayPal]

=== How to Contribute?

It's easy, just follow the contribution guidelines below:

• https://help.github.com/articles/fork-a-repo[Fork] https://github.com/rubocop/ruby-style-guide[rubocop/ruby-style-guide] on GitHub
• Make your feature addition or bug fix in a feature branch.
• Include a https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html[good description] of your changes
• Push your feature branch to GitHub
• Send a https://help.github.com/articles/using-pull-requests[Pull Request]

== Colophon

This guide is written in https://asciidoc.org/[AsciiDoc] and is published as HTML using https://asciidoctor.org/[AsciiDoctor]. The HTML version of the guide is hosted on GitHub Pages.

Originally the guide was written in Markdown, but was converted to AsciiDoc in 2019.

== License

image:https://i.creativecommons.org/l/by/3.0/88x31.png[Creative Commons License] This work is licensed under a https://creativecommons.org/licenses/by/3.0/deed.en_US[Creative Commons Attribution 3.0 Unported License]

== Spread the Word

A community-driven style guide is of little use to a community that doesn't know about its existence. Tweet about the guide, share it with your friends and colleagues. Every comment, suggestion or opinion we get makes the guide just a little bit better. And we want to have the best possible guide, don't we?