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 3 & 4 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, and 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 1.9" and "The Ruby Programming Language".

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.

The guide is still a work in progress - some rules are lacking examples, some rules don't have examples that illustrate them clearly enough. In due time these issues will be addressed - just keep them in mind for now.

You can generate a PDF or an HTML copy of this guide using Transmuter.

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

Translations of the guide are available in the following languages:

• Chinese Simplified
• Chinese Traditional
• French
• Japanese
• Spanish
• Vietnamese

Table of Contents

• Source Code Layout
• Syntax
• Naming
• Comments
  • Comment Annotations
• Classes
• Exceptions
• Collections
• Strings
• Regular Expressions
• Percent Literals
• Metaprogramming
• Misc
• Tools

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.

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

  # bad - four spaces
  def some_method
      do_something
  end
  
  # good
  def some_method
    do_something
  end

• Use Unix-style line endings. (*BSD/Solaris/Linux/OS X users are covered by default, Windows users have to be extra careful.)

  • 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.

  # bad
  puts 'foobar'; # superfluous semicolon
  
  puts 'foo'; puts 'bar' # two expression 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.

  # 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.

  # 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 ; make 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, around { and before }. Whitespace might be (mostly) irrelevant to the Ruby interpreter, but its proper use is the key to writing easily readable code.

  sum = 1 + 2
  a, b = 1, 2
  1 > 2 ? true : false; puts 'Hi'
  [1, 2, 3].each { |e| puts e }

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

  # bad
  e = M * c ** 2
  
  # good
  e = M * c**2

  { and } deserve a bit of clarification, since they are used for block and hash literals, as well as embedded expressions in strings. For hash literals two styles are considered acceptable.

  # good - space after { and before }
  { one: 1, two: 2 }
  
  # good - no space after { and before }
  {one: 1, two: 2}

  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.

  As far as embedded expressions go, there are also two acceptable options:

  # good - no spaces
  "string#{expr}"
  
  # ok - arguably more readable
  "string#{ expr }"

  The first style is extremely more popular and you're generally advised to stick with it. The second, on the other hand, is (arguably) a bit more readable. As with hashes - pick one style and apply it consistently.

• No spaces after (, [ or before ], ).

  some(arg).other
  [1, 2, 3].size

• No space after !.

  # bad
  ! something
  
  # good
  !something

• Indent when as deep as case. I know that many would disagree with this one, but it's the style established in both "The Ruby Programming Language" and "Programming 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

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

  # 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 a method into logical paragraphs internally.

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

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

  # 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:

  # 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.

  # 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).

  • (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.

  # starting point (line is too long)
  def send_mail(source)
    Mailer.deliver(to: 'bob@example.com', from: 'us@example.com', subject: 'Important message', body: source.text)
  end
  
  # bad (double indent)
  def send_mail(source)
    Mailer.deliver(
        to: 'bob@example.com',
        from: 'us@example.com',
        subject: 'Important message',
        body: source.text)
  end
  
  # good
  def send_mail(source)
    Mailer.deliver(to: 'bob@example.com',
                   from: 'us@example.com',
                   subject: 'Important message',
                   body: source.text)
  end
  
  # good (normal indent)
  def send_mail(source)
    Mailer.deliver(
      to: 'bob@example.com',
      from: 'us@example.com',
      subject: 'Important message',
      body: source.text
    )
  end

• Align the elements of array literals spanning multiple lines.

  # 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.

  # bad - how many 0s are there?
  num = 1000000
  
  # good - much easier to parse for the human brain
  num = 1_000_000

• Use RDoc and its conventions for API documentation. Don't put an empty line between the comment block and the def.

• Limit lines to 80 characters.

• Avoid trailing whitespace.

• End each file with a newline.

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

  # 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()). Never use :: for regular method invocation.

  # 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 arguments. Omit the parentheses when the method doesn't accept any arguments.

  # bad
  def some_method()
    # body omitted
  end
  
  # good
  def some_method
    # body omitted
  end
  
  # bad
  def some_method_with_arguments arg1, arg2
    # body omitted
  end
  
  # good
  def some_method_with_arguments(arg1, arg2)
    # body omitted
  end

• Never 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.

  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'

• Never use then for multi-line if/unless.

  # 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.

  # 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.

  # 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.

  # 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

• Never use if x: ... - as of Ruby 1.9 it has been removed. Use the ternary operator instead.

  # bad
  result = if some_condition: something else something_else end
  
  # good
  result = some_condition ? something : something_else

• Never use if x; .... Use the ternary operator instead.

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

  # 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.

• Never use when x; .... See the previous rule.

• Use ! instead of not.

  # bad - braces are required because of op precedence
  x = (not something)
  
  # good
  x = !something

• Avoid the use of !!.

  # bad
  x = 'test'
  # obscure nil check
  if !!x
    # body omitted
  end
  
  x = false
  # double negation is useless on booleans
  !!x # => false
  
  # good
  x = 'test'
  unless x.nil?
    # body omitted
  end

• The and and or keywords are banned. It's just not worth it. Always use && and || instead.

  # bad
  # boolean expression
  if some_condition and some_other_condition
    do_something
  end
  
  # control flow
  document.saved? or document.save!
  
  # good
  # boolean expression
  if some_condition && some_other_condition
    do_something
  end
  
  # control flow
  document.saved? || document.save!

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

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

  # bad
  if some_condition
    do_something
  end
  
  # good
  do_something if some_condition
  
  # another good option
  some_condition && do_something

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

  # 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

• Never use unless with else. Rewrite these with the positive case first.

  # 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 an if/unless/while/until.

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

• Never use while/until condition do for multi-line while/until.

  # 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.

  # bad
  while some_condition
    do_something
  end
  
  # good
  do_something while some_condition

• Favor until over while for negative conditions.

  # bad
  do_something while !some_condition
  
  # good
  do_something until some_condition

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

  # bad
  begin
    puts val
    val += 1
  end while val < 0
  
  # good
  loop do
    puts val
    val += 1
    break unless val < 0
  end

• Omit parentheses around parameters for methods that are part of an internal DSL (e.g. Rake, Rails, RSpec), methods that have "keyword" status in Ruby (e.g. attr_reader, puts) and attribute access methods. Use parentheses around the arguments of all other method invocations.

  class Person
    attr_reader :name, :age
  
    # omitted
  end
  
  temperance = Person.new('Temperance', 30)
  temperance.name
  
  puts temperance.age
  
  x = Math.sin(y)
  array.delete(e)
  
  bowling.score.should == 0

• Omit the outer braces around an implicit options hash.

  # 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.

  class Person < ActiveRecord::Base
    # bad
    validates(:name, { presence: true, length: { within: 1..10 } })
  
    # good
    validates :name, presence: true, length: { within: 1..10 }
  end

• Omit parentheses for method calls with no arguments.

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

• Prefer {...} over do...end for single-line blocks. Avoid using {...} for multi-line blocks (multiline 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.

  names = ['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 { |name| name.upcase }

  Some will argue that multiline 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.

  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 parameter and pwd is set: #{dir}"
  end

• Avoid return where not required for flow of control.

  # 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.)

  # 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.

  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.

  # bad (+ a warning)
  if v = array.grep(/foo/)
    do_something(v)
    ...
  end
  
  # good (MRI would still complain, but RuboCop won't)
  if (v = array.grep(/foo/))
    do_something(v)
    ...
  end
  
  # good
  v = array.grep(/foo/)
  if v
    do_something(v)
    ...
  end

• Use shorthand self assignment operators whenever applicable.

  # 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.

  # 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.)

  # 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.

  # bad
  if something
    something = something.downcase
  end
  
  # bad
  something = something ? nil : something.downcase
  
  # 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.

  # bad
  Array === something
  (1..100) === 7
  /something/ === some_string
  
  # good
  something.is_a?(Array)
  (1..100).include?(7)
  some_string =~ /something/

• 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.

  # bad
  $:.unshift File.dirname(__FILE__)
  
  # good
  require 'English'
  $LOAD_PATH.unshift File.dirname(__FILE__)

• Never put a space between a method name and the opening parenthesis.

  # bad
  f (3 + 2) + 1
  
  # good
  f(3 + 2) + 1

• If the first argument to a method begins with an open parenthesis, always use parentheses in the method invocation. For example, write 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!

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

  # 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

• Prefer proc over Proc.new.

  # 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.

  # 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.

  # bad
  result = hash.map { |k, v| v + 1 }
  
  def something(x)
    unused_var, used_var = something_else(x)
    # ...
  end
  
  # good
  result = hash.map { |_k, v| v + 1 }
  
  def something(x)
    _unused_var, used_var = something_else(x)
    # ...
  end
  
  # good
  result = hash.map { |_, v| v + 1 }
  
  def something(x)
    _, used_var = something_else(x)
    # ...
  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.

• 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).

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

  # bad
  '%d %d' % [20, 10]
  # => '20 10'
  
  # good
  sprintf('%d %d', 20, 10)
  # => '20 10'
  
  # good
  sprintf('%{first} %{second}', first: 20, second: 10)
  # => '20 10'
  
  format('%d %d', 20, 10)
  # => '20 10'
  
  # good
  format('%{first} %{second}', first: 20, second: 10)
  # => '20 10'

• Favor the use of Array#join over the fairly cryptic Array#* with a string argument.

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

• Use [*var] or Array() instead of explicit Array check, when dealing with a variable you want to treat as an Array, but you're not certain it's an array.

  # bad
  paths = [paths] unless paths.is_a? Array
  paths.each { |path| do_something(path) }
  
  # good
  [*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.

  # 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.

  # 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

• Avoid the use of BEGIN blocks.

• Never use END blocks. Use Kernel#at_exit instead.

  # bad
  
  END { puts 'Goodbye!' }
  
  # good
  
  at_exit { puts 'Goodbye!' }

• Avoid the use of flip-flops.

• 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.

  # bad
    def compute_thing(thing)
      if thing[:foo]
        update_with_bar(thing)
        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

Naming

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

• Name identifiers in English.

  # 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.

  # bad
  :'some symbol'
  :SomeSymbol
  :someSymbol
  
  someVar = 5
  
  def someMethod
    ...
  end
  
  def SomeMethod
   ...
  end
  
  # good
  :some_symbol
  
  def some_method
    ...
  end

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

  # bad
  class Someclass
    ...
  end
  
  class Some_Class
    ...
  end
  
  class SomeXml
    ...
  end
  
  # good
  class SomeClass
    ...
  end
  
  class SomeXML
    ...
  end

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

• 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.

• Use SCREAMING_SNAKE_CASE for other constants.

  # 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.

• 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.

  # bad - there is not 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.

  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 using reduce with short blocks, name the arguments |a, e| (accumulator, element).

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

  def +(other)
    # body omitted
  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.

• 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.

  # 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.

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

• Use reverse_each instead of reverse.each. reverse_each doesn't do a new array allocation and that's a good thing.

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

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!

• Write comments in English.

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

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

• Avoid superfluous comments.

  # bad
  counter += 1 # Increments counter by one.

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

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

Comment Annotations

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

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

• If multiple lines are required to describe the problem, subsequent lines should be indented two spaces after the #.

  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.

  def bar
    sleep 100 # OPTIMIZE
  end

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

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

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

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

• 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?

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

Classes & Modules

• Use a consistent structure in your class definitions.

  class Person
    # extend and include go first
    extend SomeModule
    include AnotherModule
  
    # 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
  
    # followed by 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

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

  # bad
  class SomeClass
    def self.some_method
      # body omitted
    end
  
    def self.some_other_method
    end
  end
  
  # good
  module SomeClass
    module_function
  
    def some_method
      # body omitted
    end
  
    def some_other_method
    end
  end

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

  # 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.

• Try to make your classes as SOLID as possible.

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

  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.

  # 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

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

  # bad - creates a single attribute accessor (deprecated in 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.

  # 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 a Struct.new - it already is a new class. Extending it introduces a superfluous class level and may also introduce weird errors if the file is required multiple times.

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

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

• Prefer duck-typing over inheritance.

  # 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

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

  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.

• 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). After all we're coding in Ruby now, not in Python.

• Indent the public, protected, and private methods as much 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.

  class SomeClass
    def public_method
      # ...
    end
  
    private
  
    def private_method
      # ...
    end
  
    def another_private_method
      # ...
    end
  end

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

  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 singleton methods.
    class << self
      def first_method
        # body omitted
      end
  
      def second_method_etc
        # body omitted
      end
    end
  end

Exceptions

• Signal exceptions using the fail method. Use raise only when catching an exception and re-raising it (because here you're not failing, but explicitly and purposefully raising an exception).

  begin
    fail 'Oops'
  rescue => error
    raise if error.message != 'Oops'
  end

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

  # bad
  fail RuntimeError, 'message'
  
  # good - signals a RuntimeError by default
  fail 'message'

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

  # bad
  fail SomeException.new('message')
  # Note that there is no way to do `fail SomeException.new('message'), backtrace`.
  
  # good
  fail SomeException, 'message'
  # Consistent with `fail SomeException, 'message', backtrace`.

• Never 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.

  def foo
    begin
      fail
    ensure
      return 'very bad idea'
    end
  end

• Use implicit begin blocks where possible.

  # 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

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

  # 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 }

• Don't suppress exceptions.

  # bad
  begin
    # an exception occurs here
  rescue SomeError
    # the rescue clause does absolutely nothing
  end
  
  # bad
  do_something rescue nil

• Avoid using rescue in its modifier form.

  # 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 => ex
    handle_error(ex)
  end

• Don't use exceptions for flow of control.

  # 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

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

  # 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

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

  # bad
  begin
    # some code
  rescue Exception => e
    # some handling
  rescue StandardError => e
    # some handling
  end
  
  # good
  begin
    # some code
  rescue StandardError => e
    # some handling
  rescue Exception => e
    # some handling
  end

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

  f = File.open('testfile')
  begin
    # .. process
  rescue
    # .. handle error
  ensure
    f.close unless f.nil?
  end

• Favor the use of exceptions for the standard library over introducing new exception classes.

Collections

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

  # bad
  arr = Array.new
  hash = Hash.new
  
  # good
  arr = []
  hash = {}

• 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.

  # bad
  STATES = ['draft', 'open', 'closed']
  
  # good
  STATES = %w(draft open closed)

• 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.

  # bad
  STATES = [:draft, :open, :closed]
  
  # good
  STATES = %i(draft open closed)

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

  # 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]

• Avoid the creation of huge gaps in arrays.

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

• When accessing the first or last element from an array, prefer first or last over [0] or [-1].

• 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.

• Prefer symbols instead of strings as hash keys.

  # bad
  hash = { 'one' => 1, 'two' => 2, 'three' => 3 }
  
  # good
  hash = { one: 1, two: 2, three: 3 }

• Avoid the use of mutable objects as hash keys.

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

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

• 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.

  # bad
  { a: 1, 'b' => 2 }
  
  # good
  { :a => 1, 'b' => 2 }

• Use Hash#key? instead of Hash#has_key? and Hash#value? instead of Hash#has_value?. As noted here by Matz, the longer forms are considered deprecated.

  # bad
  hash.has_key?(:test)
  hash.has_value?(value)
  
  # good
  hash.key?(:test)
  hash.value?(value)

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

  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)

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

  batman = { name: 'Bruce Wayne', is_evil: false }
  
  # bad - if we just use || operator with falsy value we won't get the expected result
  batman[:is_evil] || true # => true
  
  # good - fetch work correctly with falsy values
  batman.fetch(:is_evil, true) # => false

• Prefer the use of the block instead of the default value in Hash#fetch.

  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, get_batman_powers) # get_batman_powers is an expensive call
  
  # good - blocks are lazy evaluated, so only triggered in case of KeyError exception
  batman.fetch(:powers) { get_batman_powers }

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

  # bad
  email = data['email']
  nickname = data['nickname']
  
  # good
  email, username = data.values_at('email', 'nickname')

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

• Never modify a collection while traversing it.

Strings

• Prefer string interpolation and string formatting instead of string concatenation:

  # 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)

• Consider padding string interpolation code with space. It more clearly sets the code apart from the string.

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

• 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 (Option A) and double quotes by default (Option B).

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

    # bad
    name = "Bozhidar"
    
    # good
    name = 'Bozhidar'

  • (Option B) Prefer double-quotes unless your string literal contains " or escape characters you want to suppress.

    # bad
    name = 'Bozhidar'
    
    # good
    name = "Bozhidar"

  The second style is arguably a bit more popular in the Ruby community. The string literals in this guide, however, are aligned with the first style.

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

  # bad
  char = ?c
  
  # good
  char = 'c'

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

  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}"

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

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

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

  # good and also fast
  html = ''
  html << '<h1>Page title</h1>'
  
  paragraphs.each do |paragraph|
    html << "<p>#{paragraph}</p>"
  end

• When using heredocs for multi-line strings keep in mind the fact that they preserve leading whitespace. It's a good practice to employ some margin based on which to trim the excessive whitespace.

  code = <<-END.gsub(/^\s+\|/, '')
    |def test
    |  some_method
    |  other_method
    |end
  END
  #=> "def test\n  some_method\n  other_method\nend\n"

Regular Expressions

Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems.
-- Jamie Zawinski

• Don't use regular expressions if you just need plain text search in string: string['text']

• For simple constructions you can use regexp directly through string index.

  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'

• Use non-capturing groups when you don't use captured result of parentheses.

  /(first|second)/   # bad
  /(?:first|second)/ # good

• Don't use the cryptic Perl-legacy variables denoting last regexp group matches ($1, $2, etc). Use Regexp.last_match[n] instead.

  /(regexp)/ =~ string
  ...
  
  # bad
  process $1
  
  # good
  process Regexp.last_match[1]

• Avoid using numbered groups as it can be hard to track what they contain. Named groups can be used instead.

  # bad
  /(regexp)/ =~ string
  ...
  process Regexp.last_match[1]
  
  # good
  /(?<meaningful_var>regexp)/ =~ string
  ...
  process meaningful_var

• Character classes have only a few special characters you should care about: ^, -, \, ], so don't escape . or brackets in [].

• 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/).

  string = "some injection\nusername"
  string[/^username$/]   # matches
  string[/\Ausername\z/] # don't match

• Use x modifier for complex regexps. This makes them more readable and you can add some useful comments. Just be careful as spaces are ignored.

  regexp = %r{
    start         # some text
    \s            # white space char
    (group)       # first group
    (?:alt1|alt2) # some alternation
    end
  }x

• For complex replacements sub/gsub can be used with block or hash.

Percent Literals

• 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.

  # bad (no interpolation needed)
  %(<div class="text">Some text</div>)
  # should be '<div class="text">Some text</div>'
  
  # bad (no double-quotes)
  %(This is #{quality} style)
  # should be "This is #{quality} style"
  
  # bad (multiple lines)
  %(<div>\n<span class="big">#{exclamation}</span>\n</div>)
  # should be a heredoc.
  
  # good (requires interpolation, has quotes, single line)
  %(<tr><td class="name">#{name}</td>)

• Avoid %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.

  # 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?"'

• Use %r only for regular expressions matching more than one '/' character.

  # bad
  %r(\s+)
  
  # still bad
  %r(^/(.*)$)
  # should be /^\/(.*)$/
  
  # good
  %r(^/blog/2011/(.*)$)

• Avoid the use of %x unless you're going to invoke a command with backquotes in it(which is rather unlikely).

  # bad
  date = %x(date)
  
  # good
  date = `date`
  echo = %x(echo `date`)

• 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.

• Prefer () as delimiters for all % literals, except %r. Since braces often appear inside regular expressions in many scenarios a less common character like { might be a better choice for a delimiter, depending on the regexp's content.

  # bad
  %w[one two three]
  %q{"Test's king!", John said.}
  
  # good
  %w(one two three)
  %q("Test's king!", John said.)

Metaprogramming

• Avoid needless metaprogramming.

• Do not mess around in core classes when writing libraries. (Do not monkey-patch them.)

• The block form of class_eval is preferable to the string-interpolated form.

  • when you use the string-interpolated form, always supply __FILE__ and __LINE__, so that your backtraces make sense:

    class_eval 'def use_relative_model_naming?; true; end', __FILE__, __LINE__

  • define_method is preferable to class_eval{ def ... }

• 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):

  # 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}(*args, &block)       # def capitalize(*args, &block)
          to_str.#{unsafe_method}(*args, &block)  #   to_str.capitalize(*args, &block)
        end                                       # end
  
        def #{unsafe_method}!(*args)              # def capitalize!(*args)
          @dirty = true                           #   @dirty = true
          super                                   #   super
        end                                       # end
      EOT
    end
  end

• 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.launch_state = false. Consider using delegation, proxy, or define_method instead. If you must use method_missing:

  • Be sure to 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:

    # bad
    def method_missing?(meth, *args, &block)
      if /^find_by_(?<prop>.*)/ =~ meth
        # ... lots of code to do a find_by
      else
        super
      end
    end
    
    # good
    def method_missing?(meth, *args, &block)
      if /^find_by_(?<prop>.*)/ =~ meth
        find_by(prop, *args, &block)
      else
        super
      end
    end
    
    # best of all, though, would to define_method as each findable attribute is declared

Misc

• Write ruby -w safe code.

• Avoid hashes as optional parameters. Does the method do too much? (Object initializers are exceptions for this rule).

• 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.

• Avoid parameter lists longer than three or four parameters.

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

• Use module instance variables instead of global variables.

  # bad
  $foo_bar = 1
  
  #good
  module Foo
    class << self
      attr_accessor :bar
    end
  end
  
  Foo.bar = 1

• Avoid alias when alias_method will do.

• Use OptionParser for parsing complex command line options and ruby -s for trivial command line options.

• Prefer Time.now over Time.new when retrieving the current system time.

• Code in a functional way, avoiding mutation when that makes sense.

• Do not mutate arguments unless that is the purpose of the method.

• Avoid more than three levels of block nesting.

• Be consistent. In an ideal world, be consistent with these guidelines.

• Use common sense.

Tools

Here's some tools to help you automatically check Ruby code against this guide.

RuboCop

RuboCop is a Ruby code style checker based on this style guide. RuboCop already covers a significant portion of the Guide, supports both MRI 1.9 and MRI 2.0 and has good Emacs integration.

RubyMine

RubyMine's code inspections are partially based on this guide.

Contributing

Nothing written in this guide is set in stone. It's my 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!

How to Contribute?

It's easy, just follow the contribution guidelines.

License

This work is licensed under a 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?

Cheers,
Bozhidar