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.

Translations of the guide are available in the following languages:

• Chinese Simplified
• Chinese Traditional

Table of Contents

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

Source Code Layout

Nearly everybody is convinced that every style but their own is ugly and unreadable. Leave out the "but their own" and they're probably right...
-- Jerry Coffin (on indentation)

• Use UTF-8 as the source file encoding.

• Use two spaces per indentation level. No hard tabs.

  # good
  def some_method
    do_something
  end
  
  # bad - four spaces
  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

• Use spaces around operators, after commas, colons and semicolons, around { and before }. Whitespace might be (mostly) irrelevant to the Ruby interpreter, but its proper use is the key to writing easily readable code.

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

  The only exception is when using the exponent operator:

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

• No spaces after (, [ or before ], )

  some(arg).other
  
  items[1] = "Test"
  
  def do_bleh(thing1, thing2)
    # body ommitted
  end

  EXCEPTION: Array literals maye be defined with or without leading/trailing spaces

  # Both OK
  [1, 2, 3].length
  [ 1, 2, 3 ].length

• Don't use case statements in place of if/elsif/else conditionals.

  # VERY BAD, no argument to `case'
  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, the subject changes, so use an if.
  if song.name == 'Misty'
    puts 'Not again!'
  elsif song.duration > 120
    puts 'Too long!'
  elsif Time.now.hour > 21
    puts "It's too late"
  else
    song.play
  end

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

  case person
  when Admin
    puts "Hola!"
  when User
    puts "Amigo!"
  when Guest
    puts "Good to see you friend."
  else
    raise VillianError.new(person)
  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

• Use empty lines between defs and to break up a method into logical paragraphs.

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

• If a method calls argumemt list goes over 80ish characters (use your judgement), break up the arguments onto multiple lines.

  # BAD. 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
  
  # good
  def send_mail(source)
    Mailer.deliver(
      to: 'bob@example.com',
      from: 'us@example.com',
      subject: 'Important message',
      body: source.text
    )
  end
  
  # goodish (arguments aligned)
  def send_mail(source)
    Mailer.deliver(to: 'bob@example.com',
                   from: 'us@example.com',
                   subject: 'Important message',
                   body: source.text)
  end

• Add underscores to big 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.

• Keep lines fewer than 80 characters.

• Avoid trailing whitespace.

Syntax

• Use def with parentheses when there are arguments. Omit the parentheses when the method doesn't accept any arguments.

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

• Only use for in ERB templates. 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: ... - it is removed in Ruby 1.9. 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: ... is removed in 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.

  # good
  do_something if some_condition
  
  # meh
  if some_condition
    do_something
  end

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

  # bad
  do_something if !some_condition
  
  # good
  do_something unless some_condition

• 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

• Omit parentheses around parameters for methods that are part of an internal DSL (e.g. Rake, Rails, RSpec), methods that are with "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)

• 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']
  
  # good
  names.each { |name| puts name }
  
  # bad
  names.each do |name|
    puts name
  end
  
  # good
  names.select { |name| name.start_with?('S') }.map { |name| name.upcase }
  
  # bad
  names.select do |name|
    name.start_with?('S')
  end.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't 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

• 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 at all.

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

• Using the return value of = (an assignment) is ok, but surround the assignment with parentheses.

  # good - shows intended use of assignment
  if (v = array.grep(/foo/)) ...
  
  # bad
  if v = array.grep(/foo/) ...
  
  # also good - shows intended use of assignment and has correct precedence.
  if (v = self.next_value) == 'hello' ...

• Initialize all instance variables that should have initialization defaults inside the constructor. Don't defer initialization to accessors with ||= if you know that the attribute will be used. If it's possible that it'll go unused, go ahead and defer it since the fastest code is the code that never runs.

  # BAD
  class Sack
    def items
      @items ||= []
    end
  end
  
  # GOOD
  class Sack
    attr_reader :items
  
    def initialize
      @items = []
    end
  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?

• You should know the common Ruby global variables like $0 and $/. Avoid $1-9.

• 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:

  # BAD
  add (3 + 2) + 1
  
  # GOOD
  add((3 + 2) + 1)
  
  # GOOD
  add( (3 + 2) + 1 )

• The new hash literal syntax is preferred in Ruby 1.9 when your hash keys are symbols.

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

• The new lambda literal syntax is preferred in Ruby 1.9.

  # BAD
  lambda = lambda { |a, b| a + b }
  
  # GOOD
  lambda = ->(a, b) { a + b }

• Always use #call to execute a lambda:

  add = lambda { |a, b| a + b }
  
  # BAD
  add.(1, 2)
  
  # BAD
  add[1, 2]
  
  # ACCEPTABLE (use your judgement in context)
  add.call 1, 2
  
  # GOOD
  add.call(1, 2)

• Use _ for unused block parameters.

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

Naming

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

• Use snake_case for methods and variables.

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

• Use SCREAMING_SNAKE_CASE for other constants.

• 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.

• Examples of dangerous methods would be methods that mutate collections, modify the file-system, or perform operations like require that can't be easily undone.

• Don't include non-bang "safe" methods just to have them unless that makes sense in the context of your API.

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

• When defining binary operators, name the argument other.

  def +(other)
    # body omitted
  end

• Use map over collect, detect over find, select over find_all, reduce over inject and size over length.

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 concise code

• 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.

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

• Keep in mind that there's no such thing as code that's so readable you'll understand your intent a year from now.

• Also keep in mind that you should never have more comments than code. Large detailed usage examples, development guidelines and architecture discussions belong on a Wiki, not in your code. That's just begging the comments to be carelessly obsoleted.

Annotations

• The only Annotation you should ever use is # TODO: $explanation.... The point of it is to find-in-project. If it's a FIX, include a brief note of the problem, and the solution you envision (if it occurs to you).

• 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 #.

• Feel free to use sub-classification of annotations as long as they always start with TODO.

  def bar
    # TODO: BUG: 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 # TODO: OPTIMIZE
  end

Suggestions for sub-classification:

• 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

• 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

• Never use Struct where you wouldn't also consider using an OpenStruct. Do this only for "throwaway" objects. NEVER inherit from Struct.new, as it causes Superclass Mismatch errors when reloading code.

  # BAD
  class Person < Struct.new(:first_name, :last_name)
  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 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 inheritance over duck-typing. It's self documenting and enables you to guide usage with ArgumentError checks.

  # BAD
  class Duck
    def speak
      puts 'Quack! Quack'
    end
  end
  
  class Dog
    def speak
      puts 'Bau! Bau!'
    end
  end
  
  # GOOD
  class Animal
    # abstract method
    def speak
      raise NotImplementedError.new
    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
  
  class Pets
    include Enumerable
    
    def initialize
      @pets = Set.new
    end
    
    def <<(pet)
      raise ArgumentError.new("Expected +pet+ to be an Animal but was #{pet.class.name.inspect}") unless pet.is_a?(Animal)
      @pets << pet
    end
    
    def each
      @pets.each { |pet| yield pet }
    end
  end

• Prefer class instance-variables over class-variables.

  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) 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.

• Never use protected in Ruby. It doesn't mean what it does in every other language (private is actually the equivalent to protected in other languages), it'll just confuse.

• Indent the public and private methods as much the method definitions they apply to. Leave one blank line above them.

  class SomeClass
    def public_method
      # ...
    end
  
    private
    def 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. Don't use the "clever" hack of class << self.

  class TestClass
    # BAD
    def TestClass.some_method
      # body omitted
    end
  
    # BAD
    class << self
      def first_method
        # body omitted
      end
    
      def second_method_etc
        # body omitted
      end
    end
    
    # GOOD
    def self.some_other_method
      # body omitted
    end
  end

Exceptions

• Signal exceptions using the raise keyword.

  begin
    # BAD
    fail 'Oops';
    
    # GOOD
    raise FancyPantsError.new
  rescue => error
    raise if error.message != 'Oops'
  end

• Strive to always Type your exceptions so downstream users can handle them appropriately.

  # BAD
  raise "The gem #{gem.name} wasn't found!"
  
  # GOOD
  class MissingGemError < StandardError
    def initialize(gem)
      super "The gem #{gem.name} wasn't found!"
    end
  end
  
  raise MissingGemError.new(gem)

• 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 when possible.

  # bad
  def foo
    begin
      # main logic goes here
    rescue
      # failure handling goes here
    end
  end
  
  # good
  def foo
    # main logic goes here
  rescue
    # failure handling goes here
  end

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

  # bad
  begin
    something_that_might_fail
  rescue IOError
    # handle IOError
  end
  
  begin
    something_else_that_might_fail
  rescue IOError
    # handle IOError
  end
  
  # good
  def with_io_error_handling
     yield
  rescue IOError
    # handle IOError
  end
  
  with_io_error_handling { something_that_might_fail }
  
  with_io_error_handling { something_else_that_might_fail }

• As a general rule, 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 except in parse cases.

  # BAD - this catches all StandardError exceptions
  do_something rescue nil
  
  # OK - Most of the stdlib classes that implement a #parse method
  #      don't also implement a silent #try_parse method.
  @published_at = Time::parse(value) 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

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

Collections

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

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

• Avoid percent-expressions (%w).

  # BAD
  STATES = %w(draft open closed)
  
  # GOOD
  STATES = ['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 Strings instead of Symbols as hash keys unless you've carefully considered the potential of bloating your Symbol table, which is never Garbage Collected.

• Avoid the use of mutable objects as hash keys.

• The new hash literal syntax is preferred in Ruby 1.9 when your hash keys are symbols.

  # BAD
  hash = { :one => 1, :two => 2, :three => 3 }
  
  # GOOD
  hash = { one: 1, two: 2, three: 3 }

• Avoid Hash#fetch. The value isn't assigned to the key, so it can hide performance issues if your default is expensive to compute.

• 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. Use your best judgement.

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

• Always prefer double-quoted strings.

  # BAD
  name = 'Bozhidar'
  
  # GOOD
  name = "Bozhidar"

• Avoid using String#+ or String#<< when you need to construct large data chunks. Use a StringIO instead.

  # BAD
  html = ''
  html << '<h1>Page title</h1>'
  
  paragraphs.each do |paragraph|
    html << "<p>#{paragraph}</p>"
  end
  
  # GOOD - Faster, less GC thrashing since you aren't constantly
  #        expanding the size of the String.
  html = StringIO.new
  html << '<h1>Page title</h1>'
  
  paragraphs.each do |paragraph|
    html << "<p>#{paragraph}</p>"
  end

Regular Expressions

• 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 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

• Avoid %w.

  STATES = %w(draft open closed)

• Avoid %r.

  # BAD
  %r(\s+)
  
  # BAD
  %r(^/(.*)$)
  # should be /^\/(.*)$/
  
  # GOOD
  /^\/blog\/2011\/(.*)$/

• Avoid %q, %Q, %x, %s, and %W.

Metaprogramming

• Avoid needless metaprogramming.

Misc

• Write ruby -w safe code.

• Avoid hashes as optional parameters. Does the method do too much?

• 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.

• Use class instance variables instead of global variables.

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

• Avoid alias when alias_method will do.

• Use OptionParser for parsing command line options.

• Code in a functional way, avoiding mutation in all but the cases where you absolutely cannot.

• 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.

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!

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?