Prelude

Bozhidar Batsov (from the upstream original):

One thing has always bothered me as Ruby developer—Python devs 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.

This document was originally created when I, as the Technical Lead of the company which I work for, was asked by our CTO to create some internal documents describing good style and best practices for Ruby programming. I started off by building upon this existing style guide, since I concurred with many of the points in it. 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 of 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.

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 %q(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 100 characters.
Avoid trailing whitespace.
- Textmate users might want to take a look at the Uber Glory bundle.

## 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.
Avoid multi-line x ? a : b (the ternary operator), use if/unless instead.
Use when x then ... for one-line cases. The alternative syntax when x: ... is removed in Ruby 1.9. The same is true for the when x; ... syntax.
Use && and || for boolean expressions, and 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 where not required. Use parentheses around the arguments when utilizing the return value or when chaining method invocations.

class Person
  attr_reader name, age
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.
Use return freely where it is useful for clarity.
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

# only slightly better (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'

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
puts (3 + 2) + 1

# good
puts(3 + 2) + 1
```
If the first argument to a method begins with an open parenthesis, always use parentheses in the method invocation.
```
puts((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 collect over map; find over detect; select over find_all; and 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

In comments written using #, # should be followed by a space.
Comments longer than a word are capitalized and use proper punctuation and grammar. 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.

## 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 REFACTOR 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 the project's README file.

## Classes

Excepting DSL classes (Rails models & controllers, etc.), 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
    # ...
  end

  # good
  def self.some_other_method
    # ...
  end

  # Also possible and convenient when you have to define many singleton methods:
  class << self
    def first_method
      # ...
    end

    def second_method_etc
      # ...
    end
  end
end

## Exceptions

Never 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
```

For multi-line strings, prefer heredoc.

# Okay
output =  'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec posuere, nisi nec'
ouptut << 'molestie feugiat, libero lectus sollicitudin risus, vitae fermentum diam velit at'
output << foo
output << 'ante. Nullam vitae massa semper velit vestibulum scelerisque. Ut justo metus,'
output << "dictum #{bar} non congue placerat, eleifend non felis. Pellentesque erat lectus."
puts output


# Better (avoids ever storing the variable; easier to read and work with)
puts <<-OUTPUT.strip_heredoc
  Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec posuere, nisi nec molestie
  feugiat, libero lectus sollicitudin risus, vitae fermentum diam velit at #{foo} ante. Nullam
  vitae massa semper velit vestibulum scelerisque. Ut justo metus, dictum #{bar} non congue
  placerat, eleifend non felis. Pellentesque erat lectus, luctus sit amet.
OUTPUT

## Percent Literals

Use %w or %W freely.
```
STATES = %w(draft open closed)
```

Use %Q or %() freely. Remember to use %q for cases that require no interpolation.

%q(<div class="text">Some text</div>)
%(This is #{quality} style)
%Q(<div>\n<span class="big">#{exclamation}</span>\n</div>)
%[<tr><td class="name">#{name}</td>]

Avoid use of confusing delimiters with any of the % literals:

%q"non-interpereted string"
%'interepeted'
%w/not a regex/

Use %r for regular expressions matching / characters.

# bad
/^\/blog\/2011\/(.*)$/

# good
%r(^/blog/2011/(.*)$)

Prefer (), [] or {} as delimiters for all % literals.

## Misc

Write ruby -w safe code; that is, execute files with ruby -w -c to check syntax before committing.
Avoid parameter lists longer than three or four parameters.
If you really have to, add "global" methods to Kernel and make them private. Otherwise, never use global variables within Rails.

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-style literal hash syntax.

# Bad
has_many :contact_details, :order => 'priority DESC', :dependent => :destroy

# Good
has_many :contact_details, order: 'priority DESC', dependent: :destroy

Use the new lambda syntax.

def some_method(a, &b)
  a.call 'first block'
  b.call 'second block'
end

# old style
some_method lambda { |msg| puts msg } do |msg|
  puts msg
end

# new style
some_method ->(msg='default') { puts msg } do |msg|
  puts msg
end

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.
No overriding core classes when writing libraries. (No monkey patching!) Adding behaviour/classes can be acceptable.
Do not program defensively. The exception is when accepting input to the system from outside. Within the system, all is assumed to be correct.
```
# bad
def foo(arg)
  raise ArgumentError unless arg.is_a?(String)
  # ...
end
```
```
def foo(arg)
  # ...
end
```
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.

# Notes

Nothing written in this guide is set in stone. I intend this document to be a suggestion. I expect the rest of the team have their own opinions about what is ideal code style.

There are a few elements from the original that I left, here, even though they don't apply to our needs.

Feel free to change stuff to your liking!

# Spread the Word

This style guide was written by Bozhidar Batsov.

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