ruby-style-guide 0,7,9,0,6,5,0,9

A community-driven Ruby coding style guide

2 years after

Prelude

Role models are important.
-- Officer Alex J. Murphy / RoboCop

One thing has always bothered me as a Ruby developer—Python developers have a great programming style reference ([PEP-8][]) and we never got an official guide, documenting Ruby coding style and best practices. And I do believe that style matters. I also believe that a great hacker community, such as Ruby has, should be quite capable of producing this coveted document.

This guide started its life as our internal company Ruby coding guidelines (written by yours truly). At some point I decided that the work I 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.

Since the inception of the guide I'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.

By the way, if you're into Rails you might want to check out the complementary [Ruby on Rails Style Guide][rails-style-guide].

The Ruby Style Guide

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 rules. I've tried to add the rationale behind the rules (if it's omitted I've assumed it's pretty obvious).

I didn't come up with all the rules out of nowhere—they are mostly based on my extensive career as a professional software engineer, feedback and suggestions from members of the Ruby community and various highly regarded Ruby programming resources, such as ["Programming Ruby"][pickaxe] and ["The Ruby Programming Language"][trpl].

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.

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

Many projects have their own coding style guidelines (often derived from this guide). In the event of any conflicts, such project-specific guides take precedence for that project.

You can generate a PDF or an HTML copy of this guide using [Pandoc][].

[RuboCop][] is a code analyzer, based on this style guide.

Translations of the guide are available in the following languages:

Table of Contents

Source Code Layout

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...
-- Jerry Coffin (on indentation)

  • Use UTF-8 as the source file encoding. [link]

  • Use two spaces per indentation level (aka soft tabs). No hard tabs. [link]

    # bad - four spaces
    def some_method
      do_something
    end
    
    # good
    def some_method
    do_something
    end
  • Use Unix-style line endings. (*BSD/Solaris/Linux/macOS users are covered by default, Windows users have to be extra careful.) [link]

    • If you're using Git you might want to add the following configuration setting to protect your project from Windows line endings creeping in:
    $ git config --global core.autocrlf true
  • Don't use ; to separate statements and expressions. As a corollary—use one expression per line. [link]

    # bad
    puts 'foobar'; # superfluous semicolon
    
    puts 'foo'; puts 'bar' # two expressions on the same line
    
    # good
    puts 'foobar'
    
    puts 'foo'
    puts 'bar'
    
    puts 'foo', 'bar' # this applies to puts in particular
  • Prefer a single-line format for class definitions with no body. [link]

    # bad
    class FooError < StandardError
    end
    
    # okish
    class FooError < StandardError; end
    
    # good
    FooError = Class.new(StandardError)
  • 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. [link]

    # 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.

    # good
    def no_op; end
  • 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. [link]

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

    The only exception, regarding operators, is the exponent operator:

    # bad
    e = M * c ** 2
    
    # good
    e = M * c**2
  • No spaces after (, [ or before ], ). Use spaces around { and before }. [link]

    # 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.

    # 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.

    # bad
    "From: #{ user.first_name }, #{ user.last_name }"
    
    # good
    "From: #{user.first_name}, #{user.last_name}"
  • No space after !. [link]

    # bad
    ! something
    
    # good
    !something
  • No space inside range literals. [link]

    # bad
    1 .. 3
    'a' ... 'z'
    
    # good
    1..3
    'a'...'z'
  • Indent when as deep as case. This is the style established in both "The Ruby Programming Language" and "Programming Ruby". [link]

    # 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
  • When assigning the result of a conditional expression to a variable, preserve the usual alignment of its branches. [link]

    # 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
  • Use empty lines between method definitions and also to break up methods into logical paragraphs internally. [link]

    def some_method
    data = initialize(options)
    
    data.manipulate!
    
    data.result
    end
    
    def some_method
    result
    end
  • Don't use several empty lines in a row. [link]

    # bad - It has two empty lines.
    some_method
    
    some_method
    
    # good
    some_method
    
    some_method
  • Use empty lines around access modifiers. [link]

    # bad
    class Foo
    attr_reader :foo
    def foo
      # do something...
    end
    end
    
    # good
    class Foo
    attr_reader :foo
    
    def foo
      # do something...
    end
    end
  • Don't use empty lines around method, class, module, block bodies. [link]

    # bad
    class Foo
    
    def foo
    
      begin
    
        do_something do
    
          something
    
        end
    
      rescue
    
        something
    
      end
    
    end
    
    end
    
    # good
    class Foo
    def foo
      begin
        do_something do
          something
        end
      rescue
        something
      end
    end
    end
  • Avoid comma after the last parameter in a method call, especially when the parameters are not on separate lines. [link]

    # 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)
  • Use spaces around the = operator when assigning default values to method parameters: [link]

    # 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).

  • Avoid line continuation \ where not required. In practice, avoid using line continuations for anything but string concatenation. [link]

    # bad
    result = 1 - \
           2
    
    # good (but still ugly as hell)
    result = 1 \
           - 2
    
    long_string = 'First part of the long string' \
                ' and second part of the long string'
  • Adopt a consistent multi-line method chaining style. There are two popular styles in the Ruby community, both of which are considered good—leading . (Option A) and trailing . (Option B). [link]

    • (Option A) When continuing a chained method invocation on another line keep the . on the second line.
    # 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
    • (Option B) When continuing a chained method invocation on another line, include the . on the first line to indicate that the expression continues.
    # 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 here.

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

    # 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
  • Align the elements of array literals spanning multiple lines. [link]

    # bad - single indent
    menu_item = ['Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam',
    'Baked beans', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam']
    
    # good
    menu_item = [
    'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam',
    'Baked beans', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam'
    ]
    
    # good
    menu_item =
    ['Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam',
     'Baked beans', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam']
  • Add underscores to large numeric literals to improve their readability. [link]

    # bad - how many 0s are there?
    num = 1000000
    
    # good - much easier to parse for the human brain
    num = 1_000_000
  • Prefer smallcase letters for numeric literal prefixes. 0o for octal, 0x for hexadecimal and 0b for binary. Do not use 0d prefix for decimal literals. [link]

    # 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
  • Use [Rdoc][rdoc] and its conventions for API documentation. Don't put an empty line between the comment block and the def. [link]

  • Limit lines to 80 characters. [link]

  • Avoid trailing whitespace. [link]

  • End each file with a newline. [link]

  • Don't use block comments. They cannot be preceded by whitespace and are not as easy to spot as regular comments. [link]

    # bad
    =begin
    comment line
    another comment line
    =end
    
    # good
    # comment line
    # another comment line

Syntax

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

    # bad
    SomeClass::some_method
    some_object::some_method
    
    # good
    SomeClass.some_method
    some_object.some_method
    SomeModule::SomeClass::SOME_CONST
    SomeModule::SomeClass()
  • Use def with parentheses when there are parameters. Omit the parentheses when the method doesn't accept any parameters. [link]

    # 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
  • Use parentheses around the arguments of method invocations, especially if the first argument begins with an open parenthesis (, as in f((3 + 2) + 1). [link]

    # 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)

    Always omit parentheses for

    • Method calls with no arguments:
    # bad
    Kernel.exit!()
    2.even?()
    fork()
    'test'.upcase()
    
    # good
    Kernel.exit!
    2.even?
    fork
    'test'.upcase
    • Methods that are part of an internal DSL (e.g., Rake, Rails, RSpec):
    # bad
    validates(:name, presence: true)
    # good
    validates :name, presence: true
    • Methods that have "keyword" status in Ruby:
    class Person
      # bad
      attr_reader(:name, :age)
      # good
      attr_reader :name, :age
    
      # body omitted
    end

    Can omit parentheses for

    • Methods that have "keyword" status in Ruby, but are not declarative:
    # good
    puts(temperance.age)
    system('ls')
    # also good
    puts temperance.age
    system 'ls'
  • 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. [link]

    # 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'
  • Avoid the use of parallel assignment for defining variables. Parallel assignment is allowed when it is the return of a method call, used with the splat operator, or when used to swap variable assignment. Parallel assignment is less readable than separate assignment. [link]

    # 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]
  • 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. [link]

    # 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(',')
  • 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. [link]

    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's block
    elem # => NameError: undefined local variable or method `elem'
  • Do not use then for multi-line if/unless. [link]

    # bad
    if some_condition then
    # body omitted
    end
    
    # good
    if some_condition
    # body omitted
    end
  • Always put the condition on the same line as the if/unless in a multi-line conditional. [link]

    # bad
    if
    some_condition
    do_something
    do_something_else
    end
    
    # good
    if some_condition
    do_something
    do_something_else
    end
  • Favor the ternary operator(?:) over if/then/else/end constructs. It's more common and obviously more concise. [link]

    # bad
    result = if some_condition then something else something_else end
    
    # good
    result = some_condition ? something : something_else
  • 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. [link]

    # 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
  • Do not use if x; .... Use the ternary operator instead. [link]

    # bad
    result = if some_condition; something else something_else end
    
    # good
    result = some_condition ? something : something_else
  • Leverage the fact that if and case are expressions which return a result. [link]

    # bad
    if condition
    result = x
    else
    result = y
    end
    
    # good
    result =
    if condition
      x
    else
      y
    end
  • Use when x then ... for one-line cases. The alternative syntax when x: ... has been removed as of Ruby 1.9. [link]

  • Do not use when x; .... See the previous rule. [link]

  • Use ! instead of not. [link]

    # bad - parentheses are required because of op precedence
    x = (not something)
    
    # good
    x = !something
  • Avoid the use of !!. [link]

    !! 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. If you want to do a nil check, use nil? instead.

    # bad
    x = 'test'
    # obscure nil check
    if !!x
    # body omitted
    end
    
    # good
    x = 'test'
    if x
    # body omitted
    end
  • The and and or keywords are banned. The minimal added readability is just not worth the high probability of introducing subtle bugs. For boolean expressions, always use && and || instead. For flow control, use if and unless; && and || are also acceptable but less clear. [link]

    # bad
    # boolean expression
    ok = got_needed_arguments and arguments_are_valid
    
    # control flow
    document.save or fail(RuntimeError, "Failed to save document!")
    
    # good
    # boolean expression
    ok = got_needed_arguments && arguments_are_valid
    
    # control flow
    fail(RuntimeError, "Failed to save document!") unless document.save
    
    # ok
    # control flow
    document.save || fail(RuntimeError, "Failed to save document!")
  • Avoid multi-line ?: (the ternary operator); use if/unless instead. [link]

  • Favor modifier if/unless usage when you have a single-line body. Another good alternative is the usage of control flow &&/||. [link]

    # bad
    if some_condition
    do_something
    end
    
    # good
    do_something if some_condition
    
    # another good option
    some_condition && do_something
  • Avoid modifier if/unless usage at the end of a non-trivial multi-line block. [link]

    # 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
  • Avoid nested modifier if/unless/while/until usage. Favor &&/|| if appropriate. [link]

    # bad
    do_something if other_condition if some_condition
    
    # good
    do_something if some_condition && other_condition
  • Favor unless over if for negative conditions (or control flow ||). [link]

    # 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
  • Do not use unless with else. Rewrite these with the positive case first. [link]

    # bad
    unless success?
    puts 'failure'
    else
    puts 'success'
    end
    
    # good
    if success?
    puts 'success'
    else
    puts 'failure'
    end
  • Don't use parentheses around the condition of a control expression. [link]

    # bad
    if (x > 10)
    # body omitted
    end
    
    # good
    if x > 10
    # body omitted
    end

Note that there is an exception to this rule, namely safe assignment in condition.

  • Do not use while/until condition do for multi-line while/until. [link]

    # 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
  • Favor modifier while/until usage when you have a single-line body. [link]

    # bad
    while some_condition
    do_something
    end
    
    # good
    do_something while some_condition
  • Favor until over while for negative conditions. [link]

    # bad
    do_something while !some_condition
    
    # good
    do_something until some_condition
  • Use Kernel#loop instead of while/until when you need an infinite loop. [link]

    # bad
    while true
      do_something
    end
    
    until false
      do_something
    end
    
    # good
    loop do
      do_something
    end
  • Use Kernel#loop with break rather than begin/end/until or begin/end/while for post-loop tests. [link]

    # bad
    begin
    puts val
    val += 1
    end while val < 0
    
    # good
    loop do
    puts val
    val += 1
    break unless val < 0
    end
  • Omit the outer braces around an implicit options hash. [link]

    # bad
    user.set({ name: 'John', age: 45, permissions: { read: true } })
    
    # good
    user.set(name: 'John', age: 45, permissions: { read: true })
  • Omit both the outer braces and parentheses for methods that are part of an internal DSL. [link]

    class Person < ActiveRecord::Base
    # bad
    validates(:name, { presence: true, length: { within: 1..10 } })
    
    # good
    validates :name, presence: true, length: { within: 1..10 }
    end
  • Use the proc invocation shorthand when the invoked method is the only operation of a block. [link]

    # bad
    names.map { |name| name.upcase }
    
    # good
    names.map(&:upcase)
  • 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. [link]

    names = %w[Bozhidar Steve 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?

  • Consider using explicit block argument to avoid writing block literal that just passes its arguments to another block. Beware of the performance impact, though, as the block gets converted to a Proc. [link]

    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
  • Avoid return where not required for flow of control. [link]

    # bad
    def some_method(some_arr)
    return some_arr.size
    end
    
    # good
    def some_method(some_arr)
    some_arr.size
    end
  • Avoid self where not required. (It is only required when calling a self write accessor, methods named after reserved words, or overloadable operators.) [link]

    # 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
  • As a corollary, avoid shadowing methods with local variables unless they are both equivalent. [link]

    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
  • 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. [link]

    # 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
  • Use shorthand self assignment operators whenever applicable. [link]

    # 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
  • Use ||= to initialize variables only if they're not already initialized. [link]

    # bad
    name = name ? name : 'Bozhidar'
    
    # bad
    name = 'Bozhidar' unless name
    
    # good - set name to 'Bozhidar', only if it's nil or false
    name ||= 'Bozhidar'
  • Don't use ||= to initialize boolean variables. (Consider what would happen if the current value happened to be false.) [link]

    # bad - would set enabled to true even if it was false
    enabled ||= true
    
    # good
    enabled = true if enabled.nil?
  • 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. [link]

    # 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
  • 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. [link]

    # bad
    Array === something
    (1..100) === 7
    /something/ === some_string
    
    # good
    something.is_a?(Array)
    (1..100).include?(7)
    some_string =~ /something/
  • Do not use eql? when using == will do. The stricter comparison semantics provided by eql? are rarely needed in practice. [link]

    # 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
  • Avoid using Perl-style special variables (like $:, $;, etc. ). They are quite cryptic and their use in anything but one-liner scripts is discouraged. Use the human-friendly aliases provided by the English library. [link]

    # bad
    $:.unshift File.dirname(__FILE__)
    
    # good
    require 'English'
    $LOAD_PATH.unshift File.dirname(__FILE__)
  • Do not put a space between a method name and the opening parenthesis. [link]

    # bad
    f (3 + 2) + 1
    
    # good
    f(3 + 2) + 1
  • Always run the Ruby interpreter with the -w option so it will warn you if you forget either of the rules above! [link]

  • 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 invoked. [link]

    # 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
  • Use the new lambda literal syntax for single line body blocks. Use the lambda method for multi-line blocks. [link]

    # 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
  • Don't omit the parameter parentheses when defining a stabby lambda with parameters. [link]

    # bad
    l = ->x, y { something(x, y) }
    
    # good
    l = ->(x, y) { something(x, y) }
  • Omit the parameter parentheses when defining a stabby lambda with no parameters. [link]

    # bad
    l = ->() { something }
    
    # good
    l = -> { something }
  • Prefer proc over Proc.new. [link]

    # bad
    p = Proc.new { |n| puts n }
    
    # good
    p = proc { |n| puts n }
  • Prefer proc.call() over proc[] or proc.() for both lambdas and procs. [link]

    # bad - looks similar to Enumeration access
    l = ->(v) { puts v }
    l[1]
    
    # also bad - uncommon syntax
    l = ->(v) { puts v }
    l.(1)
    
    # good
    l = ->(v) { puts v }
    l.call(1)
  • 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 and will suppress their unused variable warnings. [link]

    # 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
  • 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. [link]

  • 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). [link]

  • Favor the use of sprintf and its alias format over the fairly cryptic String#% method. [link]

    # bad
    '%d %d' % [20, 10]
    # => '20 10'
    
    # good
    sprintf('%d %d', 20, 10)
    # => '20 10'
    
    # good
    sprintf('%<first>d %<second>d', first: 20, second: 10)
    # => '20 10'
    
    format('%d %d', 20, 10)
    # => '20 10'
    
    # good
    format('%<first>d %<second>d', first: 20, second: 10)
    # => '20 10'
  • When using named format string tokens, favor %<name>s over %{name} because it encodes information about the type of the value. [link]

    # bad
    format('Hello, %{name}', name: 'John')
    
    # good
    format('Hello, %<name>s', name: 'John')
  • Favor the use of Array#join over the fairly cryptic Array#* with a string argument. [link]

    # bad
    %w[one two three] * ', '
    # => 'one, two, three'
    
    # good
    %w[one two three].join(', ')
    # => 'one, two, three'
  • 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. [link]

    # 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) }
  • Use ranges or Comparable#between? instead of complex comparison logic when possible. [link]

    # bad
    do_something if x >= 1000 && x <= 2000
    
    # good
    do_something if (1000..2000).include?(x)
    
    # good
    do_something if x.between?(1000, 2000)
  • Favor the use of predicate methods to explicit comparisons with ==. Numeric comparisons are OK. [link]

    # 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
  • Don't do explicit non-nil checks unless you're dealing with boolean values. [link]

    # bad
    do_something if !something.nil?
    do_something if something != nil
    
    # good
    do_something if something
    
    # good - dealing with a boolean
    def value_set?
      [email protected]_boolean.nil?
    end
  • Avoid the use of BEGIN blocks. [link]

  • Do not use END blocks. Use Kernel#at_exit instead. [link]

    # bad
    END { puts 'Goodbye!' }
    
    # good
    at_exit { puts 'Goodbye!' }
  • Avoid the use of flip-flops. [link]

  • Avoid use of nested conditionals for flow of control. [link]

    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.

    # 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.

    # 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
  • Prefer map over collect, find over detect, select over find_all, reduce over inject 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. [link]

  • 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. [link]

    # bad
    some_hash.count
    
    # good
    some_hash.size
  • 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. [link]

    # bad
    all_songs = users.map(&:songs).flatten.uniq
    
    # good
    all_songs = users.flat_map(&:songs).uniq
  • 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. [link]

    # bad
    array.reverse.each { ... }
    
    # good
    array.reverse_each { ... }

Naming

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

  • Name identifiers in English. [link]

    # bad - identifier using non-ascii characters
    заплата = 1_000
    
    # bad - identifier is a Bulgarian word, written with Latin letters (instead of Cyrillic)
    zaplata = 1_000
    
    # good
    salary = 1_000
  • Use snake_case for symbols, methods and variables. [link]

    # bad
    :'some symbol'
    :SomeSymbol
    :someSymbol
    
    someVar = 5
    var_10  = 10
    
    def someMethod
    # some code
    end
    
    def SomeMethod
    # some code
    end
    
    # good
    :some_symbol
    
    some_var = 5
    var10    = 10
    
    def some_method
    # some code
    end
  • Do not separate numbers from letters on symbols, methods and variables. [link]

    # bad
    :some_sym_1
    
    some_var_1 = 1
    
    def some_method_1
    # some code
    end
    
    # good
    :some_sym1
    
    some_var1 = 1
    
    def some_method1
    # some code
    end
  • Use CamelCase for classes and modules. (Keep acronyms like HTTP, RFC, XML uppercase.) [link]

    # 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
  • Use snake_case for naming files, e.g. hello_world.rb. [link]

  • Use snake_case for naming directories, e.g. lib/hello_world/hello_world.rb. [link]

  • Aim to have just a single class/module per source file. Name the file name as the class/module, but replacing CamelCase with snake_case. [link]

  • Use SCREAMING_SNAKE_CASE for other constants. [link]

    # bad
    SomeConst = 5
    
    # good
    SOME_CONST = 5
  • 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. [link]

  • 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?. [link]

    # 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
  • 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. [link]

    # 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
  • Define the non-bang (safe) method in terms of the bang (dangerous) one if possible. [link]

    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
  • When defining binary operators, name the parameter other(<< and [] are exceptions to the rule, since their semantics are different). [link]

    def +(other)
    # body omitted
    end

Comments

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.
-- Steve McConnell

  • Write self-documenting code and ignore the rest of this section. Seriously! [link]

  • Write comments in English. [link]

  • Use one space between the leading # character of the comment and the text of the comment. [link]

  • Comments longer than a word are capitalized and use punctuation. Use one space after periods. [link]

  • Avoid superfluous comments. [link]

    # bad
    counter += 1 # Increments counter by one.
  • Keep existing comments up-to-date. An outdated comment is worse than no comment at all. [link]

Good code is like a good joke: it needs no explanation.
— old programmers maxim, through Russ Olsen

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

Comment Annotations

  • Annotations should usually be written on the line immediately above the relevant code. [link]

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

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

    def bar
    # FIXME: This has crashed occasionally since v3.2.1. It may
    #   be related to the BarBazUtil upgrade.
    baz(:quux)
    end
  • 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. [link]

    def bar
    sleep 100 # OPTIMIZE
    end
  • Use TODO to note missing features or functionality that should be added at a later date. [link]

  • Use FIXME to note broken code that needs to be fixed. [link]

  • Use OPTIMIZE to note slow or inefficient code that may cause performance problems. [link]

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

  • 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? [link]

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

Magic Comments

  • Place magic comments above all code and documentation. Magic comments should only go below shebangs if they are needed in your source file. [link]

    # good
    # frozen_string_literal: true
    # Some documentation about Person
    class Person
    end
    
    # bad
    # Some documentation about Person
    # frozen_string_literal: true
    class Person
    end
    # good
    #!/usr/bin/env ruby
    # frozen_string_literal: true
    App.parse(ARGV)
    
    # bad
    # frozen_string_literal: true
    #!/usr/bin/env ruby
    App.parse(ARGV)
  • Use one magic comment per line if you need multiple. [link]

    # good
    # frozen_string_literal: true
    # encoding: ascii-8bit
    
    # bad
    # -*- frozen_string_literal: true; encoding: ascii-8bit -*-
  • Separate magic comments from code and documentation with a blank line. [link]

    # good
    # frozen_string_literal: true
    
    # Some documentation for Person
    class Person
    # Some code
    end
    
    # bad
    # frozen_string_literal: true
    # Some documentation for Person
    class Person
    # Some code
    end

Classes & Modules

  • Use a consistent structure in your class definitions. [link]

    class Person
    # extend and include go first
    extend SomeModule
    include AnotherModule
    
    # inner classes
    CustomError = Class.new(StandardError)
    
    # 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
  • Split multiple mixins into separate statements. [link]

    # bad
    class Person
    include Foo, Bar
    end
    
    # good
    class Person
    # multiple mixins go in separate statements
    include Foo
    include Bar
    end
  • 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. [link]

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

    # 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
  • Favor the use of module_function over extend self when you want to turn a module's instance methods into class methods. [link]

    # 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
  • When designing class hierarchies make sure that they conform to the Liskov Substitution Principle. [link]

  • Try to make your classes as SOLID as possible. [link]

  • Always supply a proper to_s method for classes that represent domain objects. [link]

    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
  • Use the attr family of functions to define trivial accessors or mutators. [link]

    # 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
  • 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). [link]

    # 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
  • Avoid the use of attr. Use attr_reader and attr_accessor instead. [link]

    # 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
  • Consider using Struct.new, which defines the trivial accessors, constructor and comparison operators for you. [link]

    # 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 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. [link]

    # bad
    class Person < Struct.new(:first_name, :last_name)
    end
    
    # good
    Person = Struct.new(:first_name, :last_name)
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