Cognitive Complexity

Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.

A method's cognitive complexity is based on a few simple rules:

• Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
• Code is considered more complex for each "break in the linear flow of the code"
• Code is considered more complex when "flow breaking structures are nested"

Further reading

• Cognitive Complexity docs
• Cognitive Complexity: A new way of measuring understandability

This cop checks if the length a class exceeds some maximum value. Comment lines can optionally be ignored. The maximum allowed length is configurable.

This cop checks if the length of a method exceeds some maximum value. Comment lines can optionally be ignored. The maximum allowed length is configurable.

This cop checks that the ABC size of methods is not higher than the configured maximum. The ABC size is based on assignments, branches (method calls), and conditions. See http://c2.com/cgi/wiki?AbcMetric

This cop tries to produce a complexity score that's a measure of the complexity the reader experiences when looking at a method. For that reason it considers when nodes as something that doesn't add as much complexity as an if or a &&. Except if it's one of those special case/when constructs where there's no expression after case. Then the cop treats it as an if/elsif/elsif... and lets all the when nodes count. In contrast to the CyclomaticComplexity cop, this cop considers else nodes as adding complexity.

Example:

def my_method                   # 1
  if cond                       # 1
    case var                    # 2 (0.8 + 4 * 0.2, rounded)
    when 1 then func_one
    when 2 then func_two
    when 3 then func_three
    when 4..10 then func_other
    end
  else                          # 1
    do_something until a && b   # 2
  end                           # ===
end                             # 7 complexity points

This cop checks that the cyclomatic complexity of methods is not higher than the configured maximum. The cyclomatic complexity is the number of linearly independent paths through a method. The algorithm counts decision points and adds one.

An if statement (or unless or ?:) increases the complexity by one. An else branch does not, since it doesn't add a decision point. The && operator (or keyword and) can be converted to a nested if statement, and ||/or is shorthand for a sequence of ifs, so they also add one. Loops can be said to have an exit condition, so they add one.

This cop checks if the length of a method exceeds some maximum value. Comment lines can optionally be ignored. The maximum allowed length is configurable.

This cop checks if the length of a method exceeds some maximum value. Comment lines can optionally be ignored. The maximum allowed length is configurable.

This cop checks if the length of a method exceeds some maximum value. Comment lines can optionally be ignored. The maximum allowed length is configurable.

This cop checks if the length of a method exceeds some maximum value. Comment lines can optionally be ignored. The maximum allowed length is configurable.

This cop checks for excessive nesting of conditional and looping constructs.

You can configure if blocks are considered using the CountBlocks option. When set to false (the default) blocks are not counted towards the nesting level. Set to true to count blocks as well.

The maximum level of nesting allowed is configurable.

This cop checks for excessive nesting of conditional and looping constructs.

You can configure if blocks are considered using the CountBlocks option. When set to false (the default) blocks are not counted towards the nesting level. Set to true to count blocks as well.

The maximum level of nesting allowed is configurable.

This cop looks for uses of Perl-style regexp match backreferences like $1, $2, etc.

Example:

# bad
puts $1

# good
puts Regexp.last_match(1)

This cop checks for usage of comparison operators (==, >, <) to test numbers as zero, positive, or negative. These can be replaced by their respective predicate methods. The cop can also be configured to do the reverse.

The cop disregards #nonzero? as it its value is truthy or falsey, but not true and false, and thus not always interchangeable with != 0.

The cop ignores comparisons to global variables, since they are often populated with objects which can be compared with integers, but are not themselves Interger polymorphic.

Example: EnforcedStyle: predicate (default)

# bad

foo == 0
0 > foo
bar.baz > 0

# good

foo.zero?
foo.negative?
bar.baz.positive?

Example: EnforcedStyle: comparison

# bad

foo.zero?
foo.negative?
bar.baz.positive?

# good

foo == 0
0 > foo
bar.baz > 0

This cop checks for unused block arguments.

Example:

# bad

do_something do |used, unused|
  puts used
end

do_something do |bar|
  puts :foo
end

define_method(:foo) do |bar|
  puts :baz
end

Example:

#good

do_something do |used, _unused|
  puts used
end

do_something do
  puts :foo
end

define_method(:foo) do |_bar|
  puts :baz
end

This cop enforces using // or %r around regular expressions.

Example: EnforcedStyle: slashes (default)

# bad
snake_case = %r{^[\dA-Z_]+$}

# bad
regex = %r{
  foo
  (bar)
  (baz)
}x

# good
snake_case = /^[\dA-Z_]+$/

# good
regex = /
  foo
  (bar)
  (baz)
/x

Example: EnforcedStyle: percent_r

# bad
snake_case = /^[\dA-Z_]+$/

# bad
regex = /
  foo
  (bar)
  (baz)
/x

# good
snake_case = %r{^[\dA-Z_]+$}

# good
regex = %r{
  foo
  (bar)
  (baz)
}x

Example: EnforcedStyle: mixed

# bad
snake_case = %r{^[\dA-Z_]+$}

# bad
regex = /
  foo
  (bar)
  (baz)
/x

# good
snake_case = /^[\dA-Z_]+$/

# good
regex = %r{
  foo
  (bar)
  (baz)
}x

Example: AllowInnerSlashes: false (default)

# If `false`, the cop will always recommend using `%r` if one or more
# slashes are found in the regexp string.

# bad
x =~ /home\//

# good
x =~ %r{home/}

Example: AllowInnerSlashes: true

# good
x =~ /home\//

This cop checks for rescuing StandardError. There are two supported styles implicit and explicit. This cop will not register an offense if any error other than StandardError is specified.

Example: EnforcedStyle: implicit

# `implicit` will enforce using `rescue` instead of
# `rescue StandardError`.

# bad
begin
  foo
rescue StandardError
  bar
end

# good
begin
  foo
rescue
  bar
end

# good
begin
  foo
rescue OtherError
  bar
end

# good
begin
  foo
rescue StandardError, SecurityError
  bar
end

Example: EnforcedStyle: explicit (default)

# `explicit` will enforce using `rescue StandardError`
# instead of `rescue`.

# bad
begin
  foo
rescue
  bar
end

# good
begin
  foo
rescue StandardError
  bar
end

# good
begin
  foo
rescue OtherError
  bar
end

# good
begin
  foo
rescue StandardError, SecurityError
  bar
end

This cop checks for operators, variables and literals used in void context.

Example:

# bad

def some_method
  some_num * 10
  do_something
end

Example:

# bad

def some_method(some_var)
  some_var
  do_something
end

Example:

# good

def some_method
  do_something
  some_num * 10
end

Example:

# good

def some_method(some_var)
  do_something
  some_var
end

This cop checks for unused method arguments.

Example:

# bad

def some_method(used, unused, _unused_but_allowed)
  puts used
end

Example:

# good

def some_method(used, _unused, _unused_but_allowed)
  puts used
end

This cop checks for every useless assignment to local variable in every scope. The basic idea for this cop was from the warning of ruby -cw:

assigned but unused variable - foo

Currently this cop has advanced logic that detects unreferenced reassignments and properly handles varied cases such as branch, loop, rescue, ensure, etc.

Example:

# bad

def some_method
  some_var = 1
  do_something
end

Example:

# good

def some_method
  some_var = 1
  do_something(some_var)
end

This cop checks for usage of comparison operators (==, >, <) to test numbers as zero, positive, or negative. These can be replaced by their respective predicate methods. The cop can also be configured to do the reverse.

The cop disregards #nonzero? as it its value is truthy or falsey, but not true and false, and thus not always interchangeable with != 0.

The cop ignores comparisons to global variables, since they are often populated with objects which can be compared with integers, but are not themselves Interger polymorphic.

Example: EnforcedStyle: predicate (default)

# bad

foo == 0
0 > foo
bar.baz > 0

# good

foo.zero?
foo.negative?
bar.baz.positive?

Example: EnforcedStyle: comparison

# bad

foo.zero?
foo.negative?
bar.baz.positive?

# good

foo == 0
0 > foo
bar.baz > 0