A community-driven Ruby coding style guide
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README.md

README.md

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.

Table of Contents

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

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