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.

Align the parameters of a method call if they span over multiple lines.

# 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

# good (normal indent)
def send_mail(source)
  Mailer.deliver(
    to: 'bob@example.com',
    from: 'us@example.com',
    subject: 'Important message',
    body: source.text)
end

# better
def send_mail(source)
  Mailer.deliver(to: 'bob@example.com',
                 from: 'us@example.com',
                 subject: 'Important message',
                 body: source.text)
end

Use RDoc or TomDoc 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, 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

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, but surround the assignment with parenthesis.

# good - shows intented use of assignment
if (v = array.grep(/foo/)) ...

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

# also good - shows intended use of assignment and has correct precedence.
if (v = self.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.
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!

When the keys of your hash are symbols use the Ruby 1.9 hash literal syntax.

# bad
hash = { :one => 1, :two => 2 }

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

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

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! (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
```

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

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
class Person < Struct.new (:first_name, :last_name)
end

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.

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

Signal exceptions using the fail keyword. 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 form 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
```

Don't suppress exceptions.

# bad
begin
  # an exception occurs here
rescue SomeError
  # the rescue clause does absolutely nothing
end

# bad
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

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.

Use 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 object as hash keys.

Use the new 1.9 literal hash syntax in preference to the hashrocket syntax.

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

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

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}>"

Consider padding string interpolation code with space. It more clearly sets the code apart from the string.
```
"#{ 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 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
```

Regular Expressions

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 parenthesis.
```
/(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 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 %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.

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.luanch_state = false). Consider using delegation, proxy, or define_method instead. If you must use method_missing,
- be sure to also define respond_to?
- 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?
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.

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