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.

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" 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.

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

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

RuboCop is a code code analyzer (linter) and formatter, based on this style guide.

Translations of the guide are available in the following languages:

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
```
There are a few exceptions. One is the exponent operator:
```
# bad
e = M * c ** 2

# good
e = M * c**2
```
Another exception is the slash in rational literals:
```
# bad
o_scale = 1 / 48r

# good
o_scale = 1/48r
```
Another exception is the safe navigation operator:
```
# bad
foo &. bar
foo &.bar
foo&. bar

# good
foo&.bar
```
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". Historically it is derived from the fact that case and switch statements are not blocks, hence should not be indented, and the when and else keywords are labels (compiled in the C language, they are literally labels for JMP calls). ^[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: '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. ^[link]

# bad - single indent
menu_item = %w[Spam Spam Spam Spam Spam Spam Spam Spam
  Baked beans Spam Spam Spam Spam Spam]

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

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

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 lowercase 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 YARD and its conventions for API documentation. ^[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()

Do not use :: to define class methods. ^[link]

# bad
class Foo
  def self::some_method
  end
end

# good
class Foo
  def self.some_method
  end
end

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:

  ```Ruby
  # 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'

Use keyword arguments when passing boolean argument to a method. ^[link]

# bad
def some_method(bar = false)
  puts bar
end

# bad - common hack before keyword args were introduced
def some_method(options = {})
  bar = options.fetch(:bar, false)
  puts bar
end

# good
def some_method(bar: false)
  puts bar
end

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

Prefer keyword arguments over optional arguments. ^[link]

# bad
def some_method(a, b = 5, c = 1)
  # body omitted
end

# good
def some_method(a, b: 5, c: 1)
  # body omitted
end

Use keyword arguments instead of option hashes. ^[link]

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

# good
def some_method(bar: false)
  puts bar
end

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 raise("Failed to save document!")

# good
# boolean expression
ok = got_needed_arguments && arguments_are_valid

# control flow
raise("Failed to save document!") unless document.save

# ok
# control flow
document.save || raise("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.match?(/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?
  !@some_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

def someMethod
  # some code
end

def SomeMethod
  # some code
end

# good
:some_symbol

some_var = 5

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

var_10  = 10

def some_method_1
  # some code
end

# good
:some_sym1

some_var1 = 1

var10    = 10

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 and operator-alike methods, name the parameter other for operators with "symmetrical" semantics of operands. Symmetrical semantics means both sides of the operator are typically of same or coercible types. ^[link]
- operators and operator-alike methods with symmetrical semantics (the parameter should be named other): +, -, *, /, %, **, ==, >, <, |, &, ^, eql?, equal?;
- operators with non-symmetrical semantics (the parameter should not be named other): <<, [] (collection/item relations between operands), === (pattern/matchable relations);
Note that the rule should be followed only if both sides of the operator have the same semantics. Prominent exception in Ruby core is, for example, Array#*(int).
```
# good
def +(other)
  # body omitted
end

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

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

# bad
# Returns some string multiplied `other` times
def *(other)
  # body omitted
end

# good
# Returns some string multiplied `num` times
def *(num)
  # body omitted
end
```

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 in a file (except shebangs, which are discussed next).

# bad
# Some documentation about Person

# frozen_string_literal: true
class Person
end

# good
# frozen_string_literal: true

# Some documentation about Person
class Person
end

Place magic comments below shebangs when they are present in a file. ^[link]

# bad
# frozen_string_literal: true
#!/usr/bin/env ruby

App.parse(ARGV)

# good
#!/usr/bin/env ruby
# frozen_string_literal: true

App.parse(ARGV)

Use one magic comment per line if you need multiple. ^[link]

# bad
# -*- frozen_string_literal: true; encoding: ascii-8bit -*-

# good
# frozen_string_literal: true
# encoding: ascii-8bit

Separate magic comments from code and documentation with a blank line. ^[link]

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

# good
# frozen_string_literal: true

# Some documentation for Person
class Person
  # Some code
end

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

Define (and reopen) namespaced classes and modules using explicit nesting. Using the scope resolution operator can lead to surprising constant lookups due to Ruby's lexical scoping, which depends on the module nesting at the point of definition. ^[link]

module Utilities
  class Queue
  end
end

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

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

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

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

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)
```
Consider adding factory methods to provide additional sensible ways to create instances of a particular class. ^[link]
```
class Person
  def self.create(options_hash)
    # body omitted
  end
end
```

Prefer duck-typing over inheritance. ^[link]

# 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. ^[link]
```
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. ^[link]
Indent the public, protected, and private methods as much as the method definitions they apply to. Leave one blank line above the visibility modifier and one blank line below in order to emphasize that it applies to all methods below it. ^[link]
```
class SomeClass
  def public_method
    # some code
  end

  private

  def private_method
    # some code
  end

  def another_private_method
    # some code
  end
end
```

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

class TestClass
  # bad
  def TestClass.some_method
    # body omitted
  end

  # good
  def self.some_other_method
    # body omitted
  end

  # Also possible and convenient when you
  # have to define many class methods.
  class << self
    def first_method
      # body omitted
    end

    def second_method_etc
      # body omitted
    end
  end
end

Prefer alias when aliasing methods in lexical class scope as the resolution of self in this context is also lexical, and it communicates clearly to the user that the indirection of your alias will not be altered at runtime or by any subclass unless made explicit. ^[link]
```
class Westerner
  def first_name
    @names.first
  end

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

Also be aware of how Ruby handles aliases and inheritance: an alias references the method that was resolved at the time the alias was defined; it is not dispatched dynamically.
```
class Fugitive < Westerner
  def first_name
    'Nobody'
  end
end
```
In this example, Fugitive#given_name would still call the original Westerner#first_name method, not Fugitive#first_name. To override the behavior of Fugitive#given_name as well, you'd have to redefine it in the derived class.
```
class Fugitive < Westerner
  def first_name
    'Nobody'
  end

  alias given_name first_name
end
```

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

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

class Sting < Westerner
  include Mononymous
end

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

class TestClass
  # bad -- more work when class renamed/method moved
  def self.call(param1, param2)
    TestClass.new(param1).call(param2)
  end

  # bad -- more verbose than necessary
  def self.call(param1, param2)
    self.new(param1).call(param2)
  end

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

  # ...other methods...
end

Exceptions

Prefer raise over fail for exceptions. ^[link]

# bad
fail SomeException, 'message'

# good
raise SomeException, 'message'

Don't specify RuntimeError explicitly in the two argument version of raise. ^[link]

# bad
raise RuntimeError, 'message'

# good - signals a RuntimeError by default
raise 'message'

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

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

# good
raise SomeException, 'message'
# Consistent with `raise SomeException, 'message', backtrace`.

Do not return from an ensure block. If you explicitly return from a method inside an ensure block, the return will take precedence over any exception being raised, and the method will return as if no exception had been raised at all. In effect, the exception will be silently thrown away. ^[link]
```
# bad
def foo
  raise
ensure
  return 'very bad idea'
end
```

Use implicit begin blocks where possible. ^[link]

# 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). ^[link]

# 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. ^[link]

# 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. ^[link]

# 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. ^[link]

# 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. ^[link]

# 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. ^[link]

# bad
begin
  # some code
rescue StandardError => e
  # some handling
rescue IOError => e
  # some handling that will never be executed
end

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

Release external resources obtained by your program in an ensure block. ^[link]

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

Use versions of resource obtaining methods that do automatic resource cleanup when possible. ^[link]

# bad - you need to close the file descriptor explicitly
f = File.open('testfile')
# some action on the file
f.close

# good - the file descriptor is closed automatically
File.open('testfile') do |f|
  # some action on the file
end

Favor the use of exceptions from the standard library over introducing new exception classes. ^[link]

Collections

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

# bad
arr = Array.new
hash = Hash.new

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

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. ^[link]
```
# 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. ^[link]
```
# 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. ^[link]

# 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. ^[link]

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]. ^[link]
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. ^[link]

Prefer symbols instead of strings as hash keys. ^[link]

# 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. ^[link]

Use the Ruby 1.9 hash literal syntax when your hash keys are symbols. ^[link]

# 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. ^[link]
```
# 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?. ^[link]

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

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

Use Hash#each_key instead of Hash#keys.each and Hash#each_value instead of Hash#values.each. ^[link]

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

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

Use Hash#fetch when dealing with hash keys that should be present. ^[link]

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. ^[link]

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 works correctly with falsy values
batman.fetch(:is_evil, true) # => false

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

batman = { name: 'Bruce Wayne' }

# bad - if we use the default value, we eager evaluate it
# so it can slow the program down if done multiple times
batman.fetch(:powers, obtain_batman_powers) # obtain_batman_powers is an expensive call

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

Use Hash#values_at when you need to retrieve several values consecutively from a hash. ^[link]

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

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

Rely on the fact that as of Ruby 1.9 hashes are ordered. ^[link]
Do not modify a collection while traversing it. ^[link]
When accessing elements of a collection, avoid direct access via [n] by using an alternate form of the reader method if it is supplied. This guards you from calling [] on nil. ^[link]
```
# bad
Regexp.last_match[1]

# good
Regexp.last_match(1)
```

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

# bad
def awesome_things
  @awesome_things
end

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

Numbers

Use Integer to check type of an integer number. Since Fixnum is platform-dependent, checking against it will return different results on 32-bit and 64-bit machines. ^[link]
```
timestamp = Time.now.to_i

# bad
timestamp.is_a? Fixnum
timestamp.is_a? Bignum

# good
timestamp.is_a? Integer
```
Prefer to use ranges when generating random numbers instead of integers with offsets, since it clearly states your intentions. Imagine simulating a roll of a dice: ^[link]
```
# bad
rand(6) + 1

# good
rand(1..6)
```

Strings

Prefer string interpolation and string formatting instead of string concatenation: ^[link]

# 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)

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). ^[link]
- (Option A) Prefer single-quoted strings when you don't need string interpolation or special symbols such as \t, \n, ', etc.
```
# bad
name = "Bozhidar"

name = 'De\'Andre'

# good
name = 'Bozhidar'

name = "De'Andre"
```
- (Option B) Prefer double-quotes unless your string literal contains " or escape characters you want to suppress.
```
# bad
name = 'Bozhidar'

sarcasm = "I \"like\" it."

# good
name = "Bozhidar"

sarcasm = 'I "like" it.'
```
The string literals in this guide are aligned with the first style.
Don't use the character literal syntax ?x. Since Ruby 1.9 it's basically redundant—?x would be interpreted as 'x' (a string with a single character in it). ^[link]
```
# bad
char = ?c

# good
char = 'c'
```

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

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. ^[link]

# 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. ^[link]

# bad
html = ''
html += '<h1>Page title</h1>'

paragraphs.each do |paragraph|
  html += "<p>#{paragraph}</p>"
end

# good and also fast
html = ''
html << '<h1>Page title</h1>'

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

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

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

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

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

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

Use Ruby 2.3's squiggly heredocs for nicely indented multi-line strings. ^[link]

# bad - using Powerpack String#strip_margin
code = <<-RUBY.strip_margin('|')
  |def test
  |  some_method
  |  other_method
  |end
RUBY

# also bad
code = <<-RUBY
def test
  some_method
  other_method
end
RUBY

# good
code = <<~RUBY
  def test
    some_method
    other_method
  end
RUBY

Use descriptive delimiters for heredocs. Delimiters add valuable information about the heredoc content, and as an added bonus some editors can highlight code within heredocs if the correct delimiter is used. ^[link]

# bad
code = <<~END
  def foo
    bar
  end
END

# good
code = <<~RUBY
  def foo
    bar
  end
RUBY

# good
code = <<~SUMMARY
  An imposing black structure provides a connection between the past and
  the future in this enigmatic adaptation of a short story by revered
  sci-fi author Arthur C. Clarke.
SUMMARY

Date & Time

Prefer Time.now over Time.new when retrieving the current system time. ^[link]

Don't use DateTime unless you need to account for historical calendar reform—and if you do, explicitly specify the start argument to clearly state your intentions. ^[link]

# bad - uses DateTime for current time
DateTime.now

# good - uses Time for current time
Time.now

# bad - uses DateTime for modern date
DateTime.iso8601('2016-06-29')

# good - uses Date for modern date
Date.iso8601('2016-06-29')

# good - uses DateTime with start argument for historical date
DateTime.iso8601('1751-04-23', Date::ENGLAND)

Regular Expressions

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'] ^[link]

For simple constructions you can use regexp directly through string index. ^[link]

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 the captured result. ^[link]
```
# bad
/(first|second)/

# good
/(?:first|second)/
```
Don't use the cryptic Perl-legacy variables denoting last regexp group matches ($1, $2, etc). Use Regexp.last_match(n) instead. ^[link]
```
/(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. ^[link]

# bad
/(regexp)/ =~ string
# some code
process Regexp.last_match(1)

# good
/(?<meaningful_var>regexp)/ =~ string
# some code
process meaningful_var

Character classes have only a few special characters you should care about: ^, -, \, ], so don't escape . or brackets in []. ^[link]
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/). ^[link]
```
string = "some injection\nusername"
string[/^username$/]   # matches
string[/\Ausername\z/] # doesn'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. ^[link]

regexp = /
  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 a block or a hash. ^[link]

words = 'foo bar'
words.sub(/f/, 'f' => 'F') # => 'Foo bar'
words.gsub(/\w+/) { |word| word.capitalize } # => 'Foo Bar'

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. ^[link]

# 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 %() or the equivalent %q() unless you have a string with both ' and " in it. Regular string literals are more readable and should be preferred unless a lot of characters would have to be escaped in them. ^[link]

# bad
name = %q(Bruce Wayne)
time = %q(8 o'clock)
question = %q("What did you say?")

# good
name = 'Bruce Wayne'
time = "8 o'clock"
question = '"What did you say?"'
quote = %q(<p class='quote'>"What did you say?"</p>)

Use %r only for regular expressions matching at least one '/' character. ^[link]
```
# bad
%r{\s+}

# good
%r{^/(.*)$}
%r{^/blog/2011/(.*)$}
```
Avoid the use of %x unless you're going to invoke a command with backquotes in it (which is rather unlikely). ^[link]
```
# 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. ^[link]
Use the braces that are the most appropriate for the various kinds of percent literals. ^[link]
- () for string literals(%q, %Q).
- [] for array literals(%w, %i, %W, %I) as it is aligned with the standard array literals.
- {} for regexp literals(%r) since parentheses often appear inside regular expressions. That's why a less common character with { is usually the best delimiter for %r literals.
- () for all other literals (e.g. %s, %x)
```
# bad
%q{"Test's king!", John said.}

# good
%q("Test's king!", John said.)

# bad
%w(one two three)
%i(one two three)

# good
%w[one two three]
%i[one two three]

# bad
%r((\w+)-(\d+))
%r{\w{1,2}\d{2,5}}

# good
%r{(\w+)-(\d+)}
%r|\w{1,2}\d{2,5}|
```

Metaprogramming

Avoid needless metaprogramming. ^[link]
Do not mess around in core classes when writing libraries. (Do not monkey-patch them.) ^[link]
The block form of class_eval is preferable to the string-interpolated form. ^[link]
- 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): ^[link]

# from activesupport/lib/active_support/core_ext/string/output_safety.rb
UNSAFE_STRING_METHODS.each do |unsafe_method|
  if 'String'.respond_to?(unsafe_method)
    class_eval <<-EOT, __FILE__, __LINE__ + 1
      def #{unsafe_method}(*params, &block)       # def capitalize(*params, &block)
        to_str.#{unsafe_method}(*params, &block)  #   to_str.capitalize(*params, &block)
      end                                         # end

      def #{unsafe_method}!(*params)              # def capitalize!(*params)
        @dirty = true                             #   @dirty = true
        super                                     #   super
      end                                         # end
    EOT
  end
end

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: ^[link]
- 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, *params, &block)
  if /^find_by_(?<prop>.*)/ =~ meth
    # ... lots of code to do a find_by
  else
    super
  end
end

# good
def method_missing(meth, *params, &block)
  if /^find_by_(?<prop>.*)/ =~ meth
    find_by(prop, *params, &block)
  else
    super
  end
end

# best of all, though, would to define_method as each findable attribute is declared
```

Prefer public_send over send so as not to circumvent private/protected visibility. ^[link]

# We have an ActiveModel Organization that includes concern Activatable
module Activatable
  extend ActiveSupport::Concern

  included do
    before_create :create_token
  end

  private

  def reset_token
    # some code
  end

  def create_token
    # some code
  end

  def activate!
    # some code
  end
end

class Organization < ActiveRecord::Base
  include Activatable
end

linux_organization = Organization.find(...)
# BAD - violates privacy
linux_organization.send(:reset_token)
# GOOD - should throw an exception
linux_organization.public_send(:reset_token)

Prefer __send__ over send, as send may overlap with existing methods. ^[link]

require 'socket'

u1 = UDPSocket.new
u1.bind('127.0.0.1', 4913)
u2 = UDPSocket.new
u2.connect('127.0.0.1', 4913)
# Won't send a message to the receiver obj.
# Instead it will send a message via UDP socket.
u2.send :sleep, 0
# Will actually send a message to the receiver obj.
u2.__send__ ...

Misc

Write ruby -w safe code. ^[link]
Avoid hashes as optional parameters. Does the method do too much? (Object initializers are exceptions for this rule). ^[link]
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. ^[link]
Avoid parameter lists longer than three or four parameters. ^[link]
If you really need "global" methods, add them to Kernel and make them private. ^[link]

Use module instance variables instead of global variables. ^[link]

# bad
$foo_bar = 1

# good
module Foo
  class << self
    attr_accessor :bar
  end
end

Foo.bar = 1

Use OptionParser for parsing complex command line options and ruby -s for trivial command line options. ^[link]

Code in a functional way, avoiding mutation when that makes sense. ^[link]

a = []; [1, 2, 3].each { |i| a << i * 2 }   # bad
a = [1, 2, 3].map { |i| i * 2 }             # good

a = {}; [1, 2, 3].each { |i| a[i] = i * 17 }                # bad
a = [1, 2, 3].reduce({}) { |h, i| h[i] = i * 17; h }        # good
a = [1, 2, 3].each_with_object({}) { |i, h| h[i] = i * 17 } # good

Do not mutate parameters unless that is the purpose of the method. ^[link]
Avoid more than three levels of block nesting. ^[link]
Be consistent. In an ideal world, be consistent with these guidelines. ^[link]
Use common sense. ^[link]

Tools

Here are 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

The guide is still a work in progress—some rules are lacking examples, some rules don't have examples that illustrate them clearly enough. Improving such rules is a great (and simple way) to help the Ruby community!

In due time these issues will (hopefully) be addressed—just keep them in mind for now.

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

You can also support the project (and RuboCop) with financial contributions via Patreon.

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

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