Prelude

If you're into Rails you might want to check out the complementary. Ruby on Rails 3 & 4 Style Guide.

The Ruby Style Guide

Be consistent.

Source Code Layout

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

Limit lines to 80 characters.
Avoid trailing whitespace.
End each file with a newline.
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 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.

# 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 ; 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, 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, 3].each { |e| puts e }
class FooError < StandardError; end
```
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, no space after and before { and }, respectively.
```
# bad - space after { and before }
{ one: 1, two: 2 }

# good - no space after { and before }
{one: 1, two: 2}
```
As far as embedded expressions go, spaces before and after are unacceptable
```
# good - no spaces
"string#{expr}"

# bad - spaces
"string#{ expr }"
```
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

# bad - whitespace isn't free when you only have 80 characters.
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.

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

# good
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'

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

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: '[email protected]', from: '[email protected]', subject: 'Important message', body: source.text)
end

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

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

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

Align the elements of array literals spanning multiple lines.

# bad - brackets/parentheses should be on separate line
menu_item = ['Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam',
  'Baked beans', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam']

# bad
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

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

# While defining methods using the constant syntax isn't best practice, they
# exist, this is the best practice for calling them.
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 Enumerable 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.

# bad
case
when x; y
end

# good
case
when x then y
end

Use ! instead of not.

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

# good
x = !something

Avoid the use of !!. Exceptions are for referencing large objects that would stick around longer than necessary.

# bad
x = 'test'
# obscure nil check
if !!x
  # body omitted
end

x = false
# double negation is useless when you know the object is a boolean
!!x # => false

# good
x = 'test'
if x
  # body omitted
end

The and and or keywords are banned. It's just not worth it. Always use && and || instead. Use parentheses rather than depend on binding differences.

# 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

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

# 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

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

Always write the positive case or 'happy path' first. Prefer to not use unless with else.

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

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

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

# less good, but reasonable if there is no reasonable means to determine
the positive case.
# Prefer unless condition over ! condition
unless failure?
  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 instead of while/until when you need an infinite loop.

# 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.

# 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 "method definitions" (e.g. in Rakefiles and certain DSLs).

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 }

Always use an explicit block argument literal. 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 a 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.

# 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

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

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

# bad (+ a warning)
if v = array.grep(/foo/)
  do_something(v)
  ...
end

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

# bad (MRI doesn't explain)
if (v = array.grep(/foo/)).length.eq(3)
  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
something = something ? nil : something.downcase

# bad
something = something.downcase if something

# bad
something = something && something.downcase

# might be reasonable depending on length of method call
if something
  something = something.downcase
end

# good
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.

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

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

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'

# good
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

# ok
if x == 0
end

# good
if x.even?
end

if x.odd?
end

if x.nil?
end

if x.zero?
end

Don't do explicit non-nil checks unless you're dealing with boolean values.

# 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.

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

Prefer next in loops as a guard clause 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

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. Note: Ruby itself violates this.

# 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.
Use snake_case for naming directories, e.g. lib/hello_world/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 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
```
Prefer map over collect, detect over find, select over find_all, reduce over inject. 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 { ... }
```

Classes & Modules

Use a consistent structure in your class definitions.

class Person
  # constants that may be overridden by modules via prepend
  SOME_CONSTANT = 20

  # extend and include go first
  prepend SomeModule
  extend AnotherModule
  include SomeOtherModule

  # constants are next
  SOME_CONSTANT = 20

  # inner public class
  CustomErrorKlass = Class.new(StandardError)

  # 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

  def private_inner_class
    Class.new(StandardError)
  end
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.

# bad

# foo.rb
class Foo
  class Bar
    # 30 methods inside
  end

  class Car
    # 20 methods inside
  end

  # 30 methods inside
end

# good

# foo.rb
class Foo
  # 30 methods inside
end

# foo/bar.rb
class Foo
  class Bar
    # 30 methods inside
  end
end

# foo/car.rb
class Foo
  class Car
    # 20 methods inside
  end
end

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

# 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

Avoid the usage of class (@@) variables due to their 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 protected only if you actually need sibling instances to access the protected method

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
end

Exceptions

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

Don't suppress exceptions unless you can actually handle the problem.

# 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

Collections

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 when the items are not on separate lines.
```
# bad
VALUES = [1001, 2020, 3333, ]

# good
VALUES = [1001, 2020, 3333]
```
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 }

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

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')

Be aware of the fact that as of Ruby 1.9 hashes are ordered and do not re-order.
Prefer not to 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)

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).
- 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'
```

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}"

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
```

Regular Expressions

Use non-capturing groups when you don't use captured result of parentheses.
```
/(first|second)/   # bad
/(?:first|second)/ # good
```
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/] # doesn't match
```
For complex replacements sub/gsub can be used with block or hash.

Percent Literals

Prefer %r for regular expressions.
```
# bad
/(\s+)/

# good
%r|(\s+)|
```
Prefer the 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.

# 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 use string evaluation for metaprogramming! Use methods like class_exec, define_method, etc. to dynamically create dynamically named methods, anonymous modules, etc.

Misc

Write ruby -w safe code.
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.
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.

This work is licensed under a Creative Commons Attribution 3.0 Unported License

Original by Bozhidar

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