Ruby Style Guide

Prelude

This guide is copy from bbatsov's work. 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.

Table of Contents

• The Ruby Style Guide
  • Source Code Layout
  • Syntax
  • Naming
  • Comments
  • Annotations
  • Classes
  • Exceptions
  • Collections
  • Strings
  • Percent Literals
  • Miscellaneous
  • Design
• Contributing
• Spread the word

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.

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.

    # 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
    [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 "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

• 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

• 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

• 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: ... - 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. 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.

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

    # good
    if x > 10
      # 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 - it this code really readable and can't the blocks contents be extracted into
nifty methods.

• Avoid return where not required.

    # bad
    def some_method(some_arr)
      return some_arr.size
    end

    # good
    def some_method(some_arr)
      some_arr.size
    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.

    if v = array.grep(/foo/) ...

• 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!

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!, etc.) should end with an exclamation mark.
• When using inject with short blocks, name the arguments |a, e| (accumulator, element).
• When defining binary operators, name the argument other.

    def +(other)
      # body omitted
    end

• Prefer map over collect, find over detect, select over find_all, size over length. This is not a hard requirement; if the use of the alias enhances readability, it's ok to use it.

Comments

Good code is its own best documentation. As you're about to add a comment, ask yourself, "How can I improve the code so that this comment isn't needed?" Improve the code and then document it to make it even clearer.
-- Steve McConnell

• Write self-documenting code and ignore the rest of this section. Seriously!
• 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. No comment is better than an outdated comment.
• Avoid writing comments to explain bad code. Refactor the code to make it self-explanatory. (Do or do not - there is no try.)

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

    def bar
      sleep 100 # OPTIMIZE
    end

Classes

• Always supply a proper to_s method.

    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.
• Consider adding factory methods to provide additional sensible ways to create instances of a particular class.
• Prefer duck-typing over inheritance.
• Avoid the usage of class (@@) variables due to their "nasty" behavior in inheritance.
• 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 them.

    class SomeClass
      def public_method
        # ...
      end

      private
      def private_method
        # ...
      end
    end

• Use def self.method to define singleton methods. This makes the methods more resistant to refactoring changes.

    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

• Don't suppress exceptions.
• Don't use exceptions for flow of control.
• Avoid rescuing the Exception class.

Collections

• It's ok to use arrays as sets for a small number of elements.
• Prefer %w to the literal array syntax when you need an array of strings.
• Avoid the creation of huge gaps in arrays.
• Use Set instead of Array when dealing with lots of elements.
• Use symbols instead of strings as hash keys.
• Avoid the use of mutable object as hash keys.
• Use the new 1.9 literal hash syntax in preference to the hashrocket syntax.
• Rely on the fact that hashes in 1.9 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}>"

• 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 use {} around instance 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
      def to_s
        "#{@first_name} #{@last_name}"
      end

      # good
      def to_s
        "#@first_name #@last_name"
      end
    end

• 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

Percent Literals

• Use %w freely.

    STATES = %w(draft open closed)

• 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.

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.
• If you really have to, add "global" methods to Kernel and make them private.
• 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 complex command line options and ruby -s for trivial command line options.
• Write for Ruby 1.9. Don't use legacy Ruby 1.8 constructs.
  • Use the new JavaScript literal hash syntax.
  • Use the new lambda syntax.
  • Methods like inject now accept method names as arguments.

      [1, 2, 3].inject(:+)
  

• Avoid needless metaprogramming.

Design

• Code in a functional way, avoiding mutation when that makes sense.
• Do not mutate arguments unless that is the purpose of the method.
• Do not mess around in core classes when writing libraries. (Do not monkey patch them.)
• Do not program defensively.
• Keep the code simple and subjective. Each method should have a single, well-defined responsibility.
• Avoid more than three levels of block nesting.
• Don't overdesign. Overly complex solutions tend to be brittle and hard to maintain.
• Don't underdesign. A solution to a problem should be as simple as possible, but no simpler than that. Poor initial design can lead to a lot of problems in the future.
• 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?