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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.

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.

The rubocop project aims to provide an automated way to check whether a Ruby code base complies with the style guide. Currently it's far from being production ready and it's missing lots of features. Everyone is naturally invited to help improve it!

Translations of the guide are available in the following languages:

Table of Contents

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.

    # 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
      
  • Use spaces around operators, after commas, colons and semicolons. Whitespace might be (mostly) irrelevant to the Ruby interpreter, but its proper use is the key to writing easily readable code. Spacing around and within curly braces is optional.

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

    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 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

    However, something like this should generally be pulled into a method to be more clear about what's happening.

  • 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
  • Align the parameters of a method call if they span more than one line, while aligning on equals or other natural separators..

    # starting point (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
    
    # bad (double indent)
    def send_mail(source)
      Mailer.deliver(
          to: 'bob@example.com',
          from: 'us@example.com',
          subject: 'Important message',
          body: source.text)
    end
    
    # bad (ugly)
    def send_mail(source)
      Mailer.deliver(to: 'bob@example.com',
                     from: 'us@example.com',
                     subject: 'Important message',
                     body: source.text)
    end
    
    # bad (ending paranthesis needs to be on separate line)
    def send_mail(source)
      Mailer.deliver(
        to:      'bob@example.com',
        from:    'us@example.com',
        subject: 'Important message',
        body:    source.text)
    end
    
    # good (normal indent)
    def send_mail(source)
      Mailer.deliver(
        to:      'bob@example.com',
        from:    'us@example.com',
        subject: 'Important message',
        body:    source.text
      )
    end
    
    # good (normal indent with unaligned values (to save horizontal space))
    def send_mail(source)
      Mailer.deliver(
        to: 'bob@example.com',
        from: 'us@example.com',
        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.

  • Limit lines to 100 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: ... - 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. Use parentheses around the arguments of all other method invocations.

      class Person
        attr_reader :name, :age
    
        # omitted
      end
    
      temperance = Person.new('Temperance', 30)
      temperance.name
    
      puts temperance.age
    
      x = Math.sin(y)
      array.delete(e)
    
      bowling.score.should == 0
  • 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
  • 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
  • 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 }

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 = 1000
    
    # good
    salary = 1000
  • 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.

    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 self-documenting code and ignore the rest of this section. Seriously!
  • Write comments in English.
  • 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

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

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
  • 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
  • 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
  • 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 = {}
  • Prefer %w to the literal array syntax when you need an array of strings.

    # bad
    STATES = ['draft', 'open', 'closed']
    
    # good
    STATES = %w(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}>"
  • Do not pad string interpolation code with spaces. It makes it uglier.

    # bad
    "#{ user.last_name }, #{ user.first_name }"
    
    # good
    "#{user.last_name}, #{user.first_name}"
  • 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?
  • 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 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.
  • 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.

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

Creative Commons 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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