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

Checks for non-atomic file operation. And then replace it with a nearly equivalent and atomic method.

These can cause problems that are difficult to reproduce, especially in cases of frequent file operations in parallel, such as test runs with parallel_rspec.

For examples: creating a directory if there is none, has the following problems

An exception occurs when the directory didn't exist at the time of exist?, but someone else created it before mkdir was executed.

Subsequent processes are executed without the directory that should be there when the directory existed at the time of exist?, but someone else deleted it shortly afterwards.

Safety:

This cop is unsafe, because autocorrection change to atomic processing. The atomic processing of the replacement destination is not guaranteed to be strictly equivalent to that before the replacement.

Example:

# bad - race condition with another process may result in an error in `mkdir`
unless Dir.exist?(path)
  FileUtils.mkdir(path)
end

# good - atomic and idempotent creation
FileUtils.mkdir_p(path)

# bad - race condition with another process may result in an error in `remove`
if File.exist?(path)
  FileUtils.remove(path)
end

# good - atomic and idempotent removal
FileUtils.rm_f(path)

Emulates the following Ruby warnings in Ruby 2.6.

$ cat example.rb
ERB.new('hi', nil, '-', '@output_buffer')
$ ruby -rerb example.rb
example.rb:1: warning: Passing safe_level with the 2nd argument of ERB.new is
deprecated. Do not use it, and specify other arguments as keyword arguments.
example.rb:1: warning: Passing trim_mode with the 3rd argument of ERB.new is
deprecated. Use keyword argument like ERB.new(str, trim_mode:...) instead.
example.rb:1: warning: Passing eoutvar with the 4th argument of ERB.new is
deprecated. Use keyword argument like ERB.new(str, eoutvar: ...) instead.

Now non-keyword arguments other than first one are softly deprecated and will be removed when Ruby 2.5 becomes EOL. ERB.new with non-keyword arguments is deprecated since ERB 2.2.0. Use :trim_mode and :eoutvar keyword arguments to ERB.new. This cop identifies places where ERB.new(str, trim_mode, eoutvar) can be replaced by ERB.new(str, :trim_mode: trim_mode, eoutvar: eoutvar).

Example:

# Target codes supports Ruby 2.6 and higher only
# bad
ERB.new(str, nil, '-', '@output_buffer')

# good
ERB.new(str, trim_mode: '-', eoutvar: '@output_buffer')

# Target codes supports Ruby 2.5 and lower only
# good
ERB.new(str, nil, '-', '@output_buffer')

# Target codes supports Ruby 2.6, 2.5 and lower
# bad
ERB.new(str, nil, '-', '@output_buffer')

# good
# Ruby standard library style
# https://github.com/ruby/ruby/commit/3406c5d
if ERB.instance_method(:initialize).parameters.assoc(:key) # Ruby 2.6+
  ERB.new(str, trim_mode: '-', eoutvar: '@output_buffer')
else
  ERB.new(str, nil, '-', '@output_buffer')
end

# good
# Use `RUBY_VERSION` style
if RUBY_VERSION >= '2.6'
  ERB.new(str, trim_mode: '-', eoutvar: '@output_buffer')
else
  ERB.new(str, nil, '-', '@output_buffer')
end

Checks for non-atomic file operation. And then replace it with a nearly equivalent and atomic method.

These can cause problems that are difficult to reproduce, especially in cases of frequent file operations in parallel, such as test runs with parallel_rspec.

For examples: creating a directory if there is none, has the following problems

An exception occurs when the directory didn't exist at the time of exist?, but someone else created it before mkdir was executed.

Subsequent processes are executed without the directory that should be there when the directory existed at the time of exist?, but someone else deleted it shortly afterwards.

Safety:

This cop is unsafe, because autocorrection change to atomic processing. The atomic processing of the replacement destination is not guaranteed to be strictly equivalent to that before the replacement.

Example:

# bad - race condition with another process may result in an error in `mkdir`
unless Dir.exist?(path)
  FileUtils.mkdir(path)
end

# good - atomic and idempotent creation
FileUtils.mkdir_p(path)

# bad - race condition with another process may result in an error in `remove`
if File.exist?(path)
  FileUtils.remove(path)
end

# good - atomic and idempotent removal
FileUtils.rm_f(path)

Emulates the following Ruby warnings in Ruby 2.6.

$ cat example.rb
ERB.new('hi', nil, '-', '@output_buffer')
$ ruby -rerb example.rb
example.rb:1: warning: Passing safe_level with the 2nd argument of ERB.new is
deprecated. Do not use it, and specify other arguments as keyword arguments.
example.rb:1: warning: Passing trim_mode with the 3rd argument of ERB.new is
deprecated. Use keyword argument like ERB.new(str, trim_mode:...) instead.
example.rb:1: warning: Passing eoutvar with the 4th argument of ERB.new is
deprecated. Use keyword argument like ERB.new(str, eoutvar: ...) instead.

Now non-keyword arguments other than first one are softly deprecated and will be removed when Ruby 2.5 becomes EOL. ERB.new with non-keyword arguments is deprecated since ERB 2.2.0. Use :trim_mode and :eoutvar keyword arguments to ERB.new. This cop identifies places where ERB.new(str, trim_mode, eoutvar) can be replaced by ERB.new(str, :trim_mode: trim_mode, eoutvar: eoutvar).

Example:

# Target codes supports Ruby 2.6 and higher only
# bad
ERB.new(str, nil, '-', '@output_buffer')

# good
ERB.new(str, trim_mode: '-', eoutvar: '@output_buffer')

# Target codes supports Ruby 2.5 and lower only
# good
ERB.new(str, nil, '-', '@output_buffer')

# Target codes supports Ruby 2.6, 2.5 and lower
# bad
ERB.new(str, nil, '-', '@output_buffer')

# good
# Ruby standard library style
# https://github.com/ruby/ruby/commit/3406c5d
if ERB.instance_method(:initialize).parameters.assoc(:key) # Ruby 2.6+
  ERB.new(str, trim_mode: '-', eoutvar: '@output_buffer')
else
  ERB.new(str, nil, '-', '@output_buffer')
end

# good
# Use `RUBY_VERSION` style
if RUBY_VERSION >= '2.6'
  ERB.new(str, trim_mode: '-', eoutvar: '@output_buffer')
else
  ERB.new(str, nil, '-', '@output_buffer')
end