Prelude

Style is what separates the good from the great.
-- Bozhidar Batsov

One thing has always bothered me as 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 such fine fellows, like us Ruby developers, should be quite capable to produce this coveted document.

This guide started its life as our internal company Ruby coding guidelines (written by yours truly). At some point I decided that the work I was doing might be interesting to members of the Ruby community in general and that the world had little need for another internal company guideline. But the world could certainly benefit from a community-driven and community-sanctioned set of practices, idioms and style prescriptions for Ruby programming.

Since the inception of the guide I've received a lot of feedback from members of the exceptional Ruby community around the world. Thanks for all the suggestions and the support! Together we can make a resource beneficial to each and every Ruby developer out there.

By the way, if you're into Rails you might want to check out the complementary Ruby on Rails 3 Style Guide.

The Ruby Style Guide

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

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

I didn't come up with all the rules out of nowhere - they are mostly based on my extensive career as a professional software engineer, feedback and suggestions from members of the Ruby community and various highly regarded Ruby programming resources, such as "Programming Ruby 1.9" and "The Ruby Programming Language".

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

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

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

Translations of the guide are available in the following languages:

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)

Limit lines to 120 characters.
Avoid trailing whitespace.
Use UTF-8 as the source file encoding.

Use two spaces per indentation level. No hard tabs.

# bad - four spaces
def some_method
    do_something
end

# good
def some_method
  do_something
end

Use Unix-style line endings. (*BSD/Solaris/Linux/OSX users are covered by default, Windows users have to be extra careful.)
- If you're using Git you might want to add the following configuration setting to protect your project from Windows line endings creeping in:
  
  $ git config --global core.autocrlf true

Don't use ; to separate statements and expressions. As a corollary - use one expression per line.

# bad
puts 'foobar'; # superfluous semicolon

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

# good
puts 'foobar'

puts 'foo'
puts 'bar'

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

Prefer a two-line format for class definitions with no body.

# bad
class FooError < StandardError; end

# good
class FooError < StandardError
end

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 no more than one expression in a single-line method.

# bad
def too_much; something; something_else; end

# okish - notice that the first ; is required
def no_braces_method; body end

# okish - notice that the second ; is optional
def no_braces_method; body; end

# okish - valid syntax, but no ; make it kind of hard to read
def some_method() body end

# good
def some_method
  body
end

One exception to the rule are empty-body methods.

# good
def no_op; end

Use spaces around operators, after commas, colons and semicolons, around { and before }. Whitespace might be (mostly) irrelevant to the Ruby interpreter, but its proper use is the key to writing easily readable code.
```
sum = 1 + 2
a, b = 1, 2
1 > 2 ? true : false; puts 'Hi'
[1, 2, 3].each { |e| puts e }
```
{ and } deserve a bit of clarification, since they are used for block and hash literals, as well as embedded expressions in strings. For hash literals two styles are considered acceptable.
```
# good - space after { and before }
{ one: 1, two: 2 }

# good - no space after { and before }
{one: 1, two: 2}
```
The first variant is slightly more readable (and arguably more popular in the Ruby community in general). The second variant has the advantage of adding visual difference between block and hash literals. Whichever one you pick - apply it consistently.

As far as embedded expressions go, there are also two acceptable options:
```
# good - no spaces
"string#{expr}"

# bad
"string#{ expr }"
```
The first style is extremely more popular and you're generally advised to stick with it.
No spaces after (, [ or before ], ).
```
some(arg).other
[1, 2, 3].length
```

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".

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

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

Don't 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

Avoid line continuation (\) where not required. In practice, avoid using line continuations at all.

# bad
result = 1 - \
         2

# good (but still ugly as hell)
result = 1 \
         - 2

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.

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

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

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

Add underscores to large numeric literals to improve their readability.

# bad - how many 0s are there?
num = 1000000

# good - much easier to parse for the human brain
num = 1_000_000

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

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). Never use :: for method invocation.

# bad
SomeClass::some_method
some_object::some_method

# good
SomeClass.some_method
some_object.some_method
SomeModule::SomeClass::SOME_CONST

Use def without parentheses.

def some_method
  # body omitted
end

def some_method_with_arguments(arg1, arg2)
  # body omitted
end

Never use for, unless you know exactly why. Most of the time iterators should be used instead. for is implemented in terms of each (so you're adding a level of indirection), but with a twist - for doesn't introduce a new scope (unlike each) and variables defined in its block will be visible outside it.
```
arr = [1, 2, 3]

# bad
for elem in arr do
  puts elem
end

# good
arr.each { |elem| puts elem }
```

Never use then for multi-line if/unless.

# bad
if some_condition then
  # body omitted
end

# good
if some_condition
  # body omitted
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.
Use when x then ... for one-line cases. The alternative syntax when x: ... has been removed as of Ruby 1.9.
Never use when x; .... See the previous rule.
Use &&/|| for boolean expressions, and/or for control flow. (Rule of thumb: If you have to use outer parentheses, you are using the wrong operators.)
```
# boolean expression
if some_condition && some_other_condition
  do_something
end

# control flow
document.saved? or 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 and/or.

# bad
if some_condition
  do_something
end

# good
do_something if some_condition

# another good option
some_condition and do_something

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

# bad
do_something if !some_condition

# good
do_something unless some_condition

# another good option
some_condition or do_something

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

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

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

Don't use parentheses around the condition of an if/unless/while, unless the condition contains an assignment (see "Using the return value of =" below).

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

# good
if x > 10
  # body omitted
end

# ok
if (x = self.next_value)
  # body omitted
end

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

# bad
while some_condition
  do_something
end

# good
do_something while some_condition

Favor until over while for negative conditions.

# bad
do_something while !some_condition

# good
do_something until some_condition

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

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

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

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

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

Prefer {...} over do...end for single-line blocks. Avoid using {...} for multi-line blocks (multiline chaining is always ugly). Always use do...end for "control flow" and "method definitions" (e.g. in Rakefiles and certain DSLs). Avoid do...end when chaining.
```
names = ['Bozhidar', 'Steve', 'Sarah']

# bad
names.each do |name|
  puts name
end

# good
names.each { |name| puts name }

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

# good
names.select { |name| name.start_with? 'S' }.map { |name| name.upcase }
```
Some will argue that multiline chaining would look OK with the use of {...}, but they should ask themselves - is this code really readable and can the blocks' contents be extracted into nifty methods?

Avoid return where not required for flow of control.

# bad
def some_method some_arr
  return some_arr.size
end

# good
def some_method some_arr
  some_arr.size
end

Avoid self where not required. (It is only required when calling a self write accessor.)

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

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

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

class Foo
  attr_accessor :options

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

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

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

Don't use the return value of = (an assignment) in conditional expressions.

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

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

# good
v = array.grep /foo/
if v
  do_something v
  ...
end

Use ||= freely to initialize variables.

# 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?

Avoid using Perl-style special variables (like $0-9, $, etc. ). They are quite cryptic and their use in anything but one-liner scripts is discouraged.
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.

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

# good
lambda = ->(a, b) { a + b }
lambda.(1, 2)

Use _ for unused block parameters.

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

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

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 over the fairly cryptic String#% method.

# bad
'%d %d' % [20, 10]
# => '20 10'

# good
sprintf('%d %d', 20, 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 ranges instead of complex comparison logic when possible.

# bad
do_something if x >= 1000 && x < 2000

# good
do_something if (1000...2000).include?(x)

Naming

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

Name identifiers in English.

# bad - variable name written in Bulgarian with latin characters
zaplata = 1_000

# good
salary = 1_000

Use snake_case for symbols, methods and variables.

# bad
:'some symbol'
:SomeSymbol
:someSymbol

someVar = 5

def someMethod
  ...
end

def SomeMethod
 ...
end

# good
:some_symbol

def some_method
  ...
end

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

# bad
class Someclass
  ...
end

class Some_Class
  ...
end

class SomeXml
  ...
end

# good
class SomeClass
  ...
end

class SomeXML
  ...
end

Use 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?).

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

# bad - there is not matching 'safe' method
class Person
  def update!
  end
end

# good
class Person
  def update
  end
end

# good
class Person
  def update!
  end

  def update
  end
end

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

class Array
  def flatten_once!
    res = []

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

    replace(res)
  end

  def flatten_once
    dup.flatten_once!
  end
end

When using reduce with short blocks, name the arguments |a, e| (accumulator, element).
When defining binary operators, name the argument other(<< and [] are exceptions to the rule, since their semantics are different).
```
def +(other)
  # body omitted
end
```
Prefer map over collect, find over detect, select over find_all, reduce over inject and size over length. This is not a hard requirement; if the use of the alias enhances readability, it's ok to use it. The rhyming methods are inherited from Smalltalk and are not common in other programming languages. The reason the use of select is encouraged over find_all is that it goes together nicely with reject and its name is pretty self-explanatory.
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
```

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 comments in English.
Use one space between the leading # character of the comment and the text of the comment.
Comments longer than a word are capitalized and use punctuation. Use one space after periods.

Avoid superfluous comments.

# bad
counter += 1 # increments counter by one

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

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

Comment Annotations

Annotations should usually be written on the line immediately above the relevant code.
The annotation keyword is followed by a colon and a space, then a note describing the problem.

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

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

In cases where the problem is so obvious that any documentation would be redundant, annotations may be left at the end of the offending line with no note. This usage should be the exception and not the rule.
```
def bar
  sleep 100 # OPTIMIZE
end
```
Use TODO to note missing features or functionality that should be added at a later date.
Use FIXME to note broken code that needs to be fixed.
Use OPTIMIZE to note slow or inefficient code that may cause performance problems.
Use HACK to note code smells where questionable coding practices were used and should be refactored away.
Use REVIEW to note anything that should be looked at to confirm it is working as intended. For example: REVIEW: Are we sure this is how the client does X currently?
Use other custom annotation keywords if it feels appropriate, but be sure to document them in your project's README or similar.

Classes & Modules

Use a consistent structure in your class definitions.

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

  # constants are next
  SOME_CONSTANT = 20

  # afterwards we have attribute macros
  attr_reader :name

  # followed by other macros (if any)
  validates :name

  # public class methods are next in line
  def self.some_method
  end

  # followed by public instance methods
  def some_method
  end

  # protected and private methods are grouped near the end
  protected

  def some_protected_method
  end

  private

  def some_private_method
  end
end

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

# bad
class SomeClass
  def self.some_method
    # body omitted
  end

  def self.some_other_method
  end
end

# good
module SomeClass
  module_function

  def some_method
    # body omitted
  end

  def some_other_method
  end
end

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

# bad
module Utilities
  extend self

  def parse_something(string)
    # do stuff here
  end

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

# good
module Utilities
  module_function

  def parse_something(string)
    # do stuff here
  end

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

When designing class hierarchies make sure that they conform to the Liskov Substitution Principle.
Try to make your classes as [SOLID](http://en.wikipedia.org/wiki/SOLID_(object-oriented_design\)) as possible.

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

class Person
  attr_reader :first_name, :last_name

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

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

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

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

  def first_name
    @first_name
  end

  def last_name
    @last_name
  end
end

# good
class Person
  attr_reader :first_name, :last_name

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

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

# 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

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

Don't extend a Struct.new - it already is a new class. Extending it introduces a superfluous class level and may also introduce weird errors if the file is required multiple times.
Consider adding factory methods to provide additional sensible ways to create instances of a particular class.
```
class Person
  def self.create(options_hash)
    # body omitted
  end
end
```

Prefer duck-typing over inheritance.

# bad
class Animal
  # abstract method
  def speak
  end
end

# extend superclass
class Duck < Animal
  def speak
    puts 'Quack! Quack'
  end
end

# extend superclass
class Dog < Animal
  def speak
    puts 'Bau! Bau!'
  end
end

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

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

Avoid the usage of class (@@) variables due to their "nasty" behavior in inheritance.
```
class Parent
  @@class_var = 'parent'

  def self.print_class_var
    puts @@class_var
  end
end

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

Parent.print_class_var # => will print "child"
```
As you can see all the classes in a class hierarchy actually share one class variable. Class instance variables should usually be preferred over class variables.
Assign proper visibility levels to methods (private, protected) in accordance with their intended usage. Don't go off leaving everything public (which is the default). After all we're coding in Ruby now, not in Python.
Indent the public, protected, and private methods as much the method definitions they apply to. Leave one blank line above the visibility modifier and one blank line below in order to emphasize that it applies to all methods below it.
```
class SomeClass
  def public_method
    # ...
  end

  private

  def private_method
    # ...
  end

  def another_private_method
    # ...
  end
end
```

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

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

  # good
  def self.some_other_method
    # body omitted
  end

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

    def second_method_etc
      # body omitted
    end
  end
end

Exceptions

Signal exceptions using the fail method. Use raise only when catching an exception and re-raising it (because here you're not failing, but explicitly and purposefully raising an exception).
```
begin
  fail 'Oops';
rescue => error
  raise if error.message != 'Oops'
end
```
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
```

UNDER CONSIDERATION

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

UNDER CONSIDERATION

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

# bad
begin
  something_that_might_fail
rescue IOError
  # handle IOError
end

begin
  something_else_that_might_fail
rescue IOError
  # handle IOError
end

# good
def with_io_error_handling
   yield
rescue IOError
  # handle IOError
end

with_io_error_handling { something_that_might_fail }

with_io_error_handling { something_else_that_might_fail }

Don't suppress exceptions.

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

# bad
do_something rescue nil

Avoid using rescue in its modifier form.

# bad - this catches all StandardError exceptions
do_something rescue nil

Don't use exceptions for flow of control.

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

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

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

# bad
begin
  # calls to exit and kill signals will be caught (except kill -9)
  exit
rescue Exception
  puts "you didn't really want to exit, right?"
  # exception handling
end

# good
begin
  # a blind rescue rescues from StandardError, not Exception as many
  # programmers assume.
rescue => e
  # exception handling
end

# also good
begin
  # an exception occurs here

rescue StandardError => e
  # exception handling
end

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

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

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

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

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

Collections

Prefer literal array and hash creation notation (unless you need to pass parameters to their constructors, that is).
```
# bad
arr = Array.new
hash = Hash.new

# good
arr = []
hash = {}
```
Prefer %w to the literal array syntax when you need an array of words(non-empty strings without spaces and special characters in them). Apply this rule only to arrays with two or more elements.
```
# bad
STATES = ['draft', 'open', 'closed']

# good
STATES = %w(draft open closed)
```
Prefer %i to the literal array syntax when you need an array of symbols(and you don't need to maintain Ruby 1.9 compatibility). Apply this rule only to arrays with two or more elements.
```
# bad
STATES = [:draft, :open, :closed]

# good
STATES = %i(draft open closed)
```

Avoid the creation of huge gaps in arrays.

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

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

Prefer symbols instead of strings as hash keys.

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

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

Avoid the use of mutable objects as hash keys.

Use the 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}

Use 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)

Use fetch with second argument to set a default value

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

Rely on the fact that as of Ruby 1.9 hashes are ordered.
Never modify a collection while traversing it.

Strings

Prefer string interpolation instead of string concatenation:

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

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

Consider padding string interpolation code with space. It more clearly sets the code apart from the string.
```
"#{ user.last_name }, #{ user.first_name }"
```

UNDER CONSIDERATION

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

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

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

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

match = string[/regexp/]             # get content of matched regexp
first_group = string[/text(grp)/, 1] # get content of captured group
string[/text (grp)/, 1] = 'replace'  # string => 'text replace'

Use non-capturing groups when you don't use captured result of parentheses.
```
/(first|second)/   # bad
/(?:first|second)/ # good
```

Avoid using $1-9 as it can be hard to track what they contain. Named groups can be used instead.

# bad
/(regexp)/ =~ string
...
process $1

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

Character classes have only a few special characters you should care about: ^, -, \, ], so don't escape . or brackets in [].
Be careful with ^ and $ as they match start/end of line, not string endings. If you want to match the whole string use: \A and \z (not to be confused with \Z which is the equivalent of /\n?\z/).
```
string = "some injection\nusername"
string[/^username$/]   # matches
string[/\Ausername\z/] # don't match
```

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

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

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

Percent Literals

Use %() for single-line strings which require both interpolation and embedded double-quotes. For multi-line strings, prefer heredocs.

# bad (no interpolation needed)
%(<div class="text">Some text</div>)
# should be '<div class="text">Some text</div>'

# bad (no double-quotes)
%(This is #{quality} style)
# should be "This is #{quality} style"

# bad (multiple lines)
%(<div>\n<span class="big">#{exclamation}</span>\n</div>)
# should be a heredoc.

# good (requires interpolation, has quotes, single line)
%(<tr><td class="name">#{name}</td>)

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

# bad
%r(\s+)

# still bad
%r(^/(.*)$)
# should be /^\/(.*)$/

# good
%r(^/blog/2011/(.*)$)

Avoid %q, %Q, %x, %s, and %W.
Prefer () as delimiters for all % literals.

Metaprogramming

Avoid needless metaprogramming.
Do not mess around in core classes when writing libraries. (Do not monkey-patch them.)
The block form of class_eval is preferable to the string-interpolated form.
- when you use the string-interpolated form, always supply __FILE__ and __LINE__, so that your backtraces make sense:
```
class_eval 'def use_relative_model_naming?; true; end', __FILE__, __LINE__
```
- define_method is preferable to class_eval{ def ... }

When using class_eval (or other eval) with string interpolation, add a comment block showing its appearance if interpolated (a practice I learned from the Rails code):

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

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

avoid using method_missing for metaprogramming. Backtraces become messy; the behavior is not listed in #methods; misspelled method calls might silently work (nukes.launch_state = false). Consider using delegation, proxy, or define_method instead. If you must, use method_missing,
- be sure to also define respond_to_missing?
- only catch methods with a well-defined prefix, such as find_by_* -- make your code as assertive as possible.
- call super at the end of your statement
- delegate to assertive, non-magical methods:
```
# bad
def method_missing?(meth, *args, &block)
  if /^find_by_(?<prop>.*)/ =~ meth
    # ... lots of code to do a find_by
  else
    super
  end
end

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

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

Misc

Write ruby -w safe code.
Avoid hashes as optional parameters. Does the method do too much? (Object initializers are exceptions for this rule).
Avoid methods longer than 10 LOC (lines of code). Ideally, most methods will be shorter than 5 LOC. Empty lines do not contribute to the relevant LOC.
Avoid parameter lists longer than three or four parameters.
If you really need "global" methods, add them to Kernel and make them private.

Use module instance variables instead of global variables.

# bad
$foo_bar = 1

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

Foo.bar = 1

Avoid alias when alias_method will do.
Use OptionParser for parsing complex command line options and ruby -s for trivial command line options.
Code in a functional way, avoiding mutation when that makes sense.
Do not mutate arguments unless that is the purpose of the method.
Avoid more than three levels of block nesting.
Be consistent. In an ideal world, be consistent with these guidelines.
Use common sense.

Tools

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

RuboCop

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

RubyMine

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

Contributing

Nothing written in this guide is set in stone. It's my desire to work together with everyone interested in Ruby coding style, so that we could ultimately create a resource that will be beneficial to the entire Ruby community.

Feel free to open tickets or send pull requests with improvements. Thanks in advance for your help!

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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UNDER CONSIDERATION

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