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Ruby Style Guide

  • Use soft-tabs with a two space indent.

  • Keep each line of code to a readable length. Unless you have a reason to, keep lines to fewer than 100 characters.

  • Never leave trailing whitespace.

  • End each file with a newline.

  • Use spaces around operators, after commas, colons and semicolons, around { and before }.

sum = 1 + 2
a, b = 1, 2
1 > 2 ? true : false; puts "Hi"
[1, 2, 3].each { |e| puts e }
  • No spaces after (, [ or before ], ).
some(arg).other
[1, 2, 3].length
  • No spaces after !.
!array.include?(element)
  • Indent when as deep as 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

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

Classes

  • Avoid the usage of class (@@) variables due to their unusual 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.
  • 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
  • Avoid class << self except when necessary, e.g. single accessors and aliased attributes.
class TestClass
  # bad
  class << self
    def first_method
      # body omitted
    end

    def second_method_etc
      # body omitted
    end
  end

  # good
  class << self
    attr_accessor :per_page
    alias_method :nwo, :find_by_name_with_owner
  end

  def self.first_method
    # body omitted
  end

  def self.second_method_etc
    # body omitted
  end
end
  • 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
  • Avoid explicit use of self as the recipient of internal class or instance messages unless to specify a method shadowed by a variable.
class SomeClass
  attr_accessor :message

  def greeting(name)
    message = "Hi #{name}" # local variable in Ruby, not attribute writer
    self.message = message
  end
end

Collections

  • 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)

  • 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.

  • Use symbols instead of strings as hash keys.

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

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

Documentation

Use TomDoc to the best of your ability. It's pretty sweet:

# Public: Duplicate some text an arbitrary number of times.
#
# text  - The String to be duplicated.
# count - The Integer number of times to duplicate the text.
#
# Examples
#
#   multiplex("Tom", 4)
#   # => "TomTomTomTom"
#
# Returns the duplicated String.
def multiplex(text, count)
  text * count
end

Exceptions

  • 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
  • Rescue specific exceptions, not StandardError or its superclasses.
# bad
begin
  # an exception occurs here
rescue
  # exception handling
end

# still bad
begin
  # an exception occurs here
rescue Exception
  # exception handling
end

Hashes

Use the Ruby 1.9 syntax for hash literals when all the keys are symbols:

# good
user = {
  login: "defunkt",
  name: "Chris Wanstrath"
}

# bad
user = {
  :login => "defunkt",
  :name => "Chris Wanstrath"
}

Use the 1.9 syntax when calling a method with Hash options arguments or named arguments:

# good
user = User.create(login: "jane")
link_to("Account", controller: "users", action: "show", id: user)

# bad
user = User.create(:login => "jane")
link_to("Account", :controller => "users", :action => "show", :id => user)

If you have a hash with mixed key types, use the legacy hashrocket style to avoid mixing styles within the same hash:

# good
hsh = {
  :user_id => 55,
  "followers-count" => 1000
}

# bad
hsh = {
  user_id: 55,
  "followers-count" => 1000
}

Keyword Arguments

Keyword arguments are recommended but not required when a method's arguments may otherwise be opaque or non-obvious when called. Additionally, prefer them over the old "Hash as pseudo-named args" style from pre-2.0 ruby.

So instead of this:

def remove_member(user, skip_membership_check=false)
  # ...
end

# Elsewhere: what does true mean here?
remove_member(user, true)

Do this, which is much clearer.

def remove_member(user, skip_membership_check: false)
  # ...
end

# Elsewhere, now with more clarity:
remove_member user, skip_membership_check: true

Naming

  • 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. Bang methods should only exist if a non-bang method exists. (More on this).

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/(.*)$)

Regular Expressions

  • 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
  • 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.
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

Requires

Always require dependencies used directly in a script at the start of the same file. Resources that will get autoloaded on first use—such as Rails models, controllers, or helpers—don't need to be required.

require "set"
require "time"

%w(foo bar).to_set
Time.parse("2015-10-21")

This not only loads the necessary dependencies if they haven't already, but acts as documentation about the libraries that the current file uses.

Strings

  • Prefer string interpolation instead of string concatenation:
# bad
email_with_name = user.name + " <" + user.email + ">"

# good
email_with_name = "#{user.name} <#{user.email}>"
  • Use double-quoted strings. Interpolation and escaped characters will always work without a delimiter change, and ' is a lot more common than " in string literals.
# bad
name = 'Bozhidar'

# good
name = "Bozhidar"
  • 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

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
  • Avoid the ternary operator (?:) except in cases where all expressions are extremely trivial. However, do use the ternary operator(?:) over if/then/else/end constructs for single line conditionals.
# 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

  • The and and or keywords are banned. It's just not worth it. Always use && and || instead.

  • Avoid multi-line ?: (the ternary operator), use if/unless instead.

  • Favor modifier if/unless usage when you have a single-line body.

# bad
if some_condition
  do_something
end

# good
do_something if 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.
# bad
if (x > 10)
  # body omitted
end

# good
if x > 10
  # body omitted
end
  • 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 block's 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).

  • Using the return value of = (an assignment) is ok.
# bad
if (v = array.grep(/foo/)) ...

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

# also good - has correct precedence.
if (v = next_value) == "hello" ...
  • 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. Prefer long form versions such as $PROGRAM_NAME.

  • 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).

  • Use _ for unused block parameters.

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

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

  • Don't use the === (threequals) operator to check types. === is mostly an implementation detail to support Ruby features like case, and it's not commutative. For example, String === "hi" is true and "hi" === String is false. Instead, use is_a? or kind_of? if you must.

    Refactoring is even better. It's worth looking hard at any code that explicitly checks types.