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

Limit all lines to a maximum of 79 characters.

There are still many devices around that are limited to 80 character
lines; plus, limiting windows to 80 characters makes it possible to
have several windows side-by-side.  The default wrapping on such
devices looks ugly.  Therefore, please limit all lines to a maximum
of 79 characters. For flowing long blocks of text (docstrings or
comments), limiting the length to 72 characters is recommended.

Reports error E501.

Separate inline comments by at least two spaces.

An inline comment is a comment on the same line as a statement.
Inline comments should be separated by at least two spaces from the
statement. They should start with a # and a single space.

Each line of a block comment starts with a # and a single space
(unless it is indented text inside the comment).

Okay: x = x + 1  # Increment x
Okay: x = x + 1    # Increment x
Okay: # Block comment
E261: x = x + 1 # Increment x
E262: x = x + 1  #Increment x
E262: x = x + 1  #  Increment x
E265: #Block comment
E266: ### Block comment

Limit all lines to a maximum of 79 characters.

There are still many devices around that are limited to 80 character
lines; plus, limiting windows to 80 characters makes it possible to
have several windows side-by-side.  The default wrapping on such
devices looks ugly.  Therefore, please limit all lines to a maximum
of 79 characters. For flowing long blocks of text (docstrings or
comments), limiting the length to 72 characters is recommended.

Reports error E501.

Each comma, semicolon or colon should be followed by whitespace.

Okay: [a, b]
Okay: (3,)
Okay: a[1:4]
Okay: a[:4]
Okay: a[1:]
Okay: a[1:4:2]
E231: ['a','b']
E231: foo(bar,baz)
E231: [{'a':'b'}]

Limit all lines to a maximum of 79 characters.

There are still many devices around that are limited to 80 character
lines; plus, limiting windows to 80 characters makes it possible to
have several windows side-by-side.  The default wrapping on such
devices looks ugly.  Therefore, please limit all lines to a maximum
of 79 characters. For flowing long blocks of text (docstrings or
comments), limiting the length to 72 characters is recommended.

Reports error E501.

Limit all lines to a maximum of 79 characters.

There are still many devices around that are limited to 80 character
lines; plus, limiting windows to 80 characters makes it possible to
have several windows side-by-side.  The default wrapping on such
devices looks ugly.  Therefore, please limit all lines to a maximum
of 79 characters. For flowing long blocks of text (docstrings or
comments), limiting the length to 72 characters is recommended.

Reports error E501.

Continuation lines indentation.

Continuation lines should align wrapped elements either vertically
using Python's implicit line joining inside parentheses, brackets
and braces, or using a hanging indent.

When using a hanging indent these considerations should be applied:
- there should be no arguments on the first line, and
- further indentation should be used to clearly distinguish itself
  as a continuation line.

Okay: a = (\n)
E123: a = (\n    )

Okay: a = (\n    42)
E121: a = (\n   42)
E122: a = (\n42)
E123: a = (\n    42\n    )
E124: a = (24,\n     42\n)
E125: if (\n    b):\n    pass
E126: a = (\n        42)
E127: a = (24,\n      42)
E128: a = (24,\n    42)
E129: if (a or\n    b):\n    pass
E131: a = (\n    42\n 24)

Continuation lines indentation.

Continuation lines should align wrapped elements either vertically
using Python's implicit line joining inside parentheses, brackets
and braces, or using a hanging indent.

When using a hanging indent these considerations should be applied:
- there should be no arguments on the first line, and
- further indentation should be used to clearly distinguish itself
  as a continuation line.

Okay: a = (\n)
E123: a = (\n    )

Okay: a = (\n    42)
E121: a = (\n   42)
E122: a = (\n42)
E123: a = (\n    42\n    )
E124: a = (24,\n     42\n)
E125: if (\n    b):\n    pass
E126: a = (\n        42)
E127: a = (24,\n      42)
E128: a = (24,\n    42)
E129: if (a or\n    b):\n    pass
E131: a = (\n    42\n 24)

Don't use spaces around the '=' sign in function arguments.

Don't use spaces around the '=' sign when used to indicate a
keyword argument or a default parameter value, except when
using a type annotation.

Okay: def complex(real, imag=0.0):
Okay: return magic(r=real, i=imag)
Okay: boolean(a == b)
Okay: boolean(a != b)
Okay: boolean(a <= b)
Okay: boolean(a >= b)
Okay: def foo(arg: int = 42):
Okay: async def foo(arg: int = 42):

E251: def complex(real, imag = 0.0):
E251: return magic(r = real, i = imag)
E252: def complex(real, image: float=0.0):

Separate inline comments by at least two spaces.

An inline comment is a comment on the same line as a statement.
Inline comments should be separated by at least two spaces from the
statement. They should start with a # and a single space.

Each line of a block comment starts with a # and a single space
(unless it is indented text inside the comment).

Okay: x = x + 1  # Increment x
Okay: x = x + 1    # Increment x
Okay: # Block comment
E261: x = x + 1 # Increment x
E262: x = x + 1  #Increment x
E262: x = x + 1  #  Increment x
E265: #Block comment
E266: ### Block comment

Limit all lines to a maximum of 79 characters.

There are still many devices around that are limited to 80 character
lines; plus, limiting windows to 80 characters makes it possible to
have several windows side-by-side.  The default wrapping on such
devices looks ugly.  Therefore, please limit all lines to a maximum
of 79 characters. For flowing long blocks of text (docstrings or
comments), limiting the length to 72 characters is recommended.

Reports error E501.

Each comma, semicolon or colon should be followed by whitespace.

Okay: [a, b]
Okay: (3,)
Okay: a[1:4]
Okay: a[:4]
Okay: a[1:]
Okay: a[1:4:2]
E231: ['a','b']
E231: foo(bar,baz)
E231: [{'a':'b'}]

Each comma, semicolon or colon should be followed by whitespace.

Okay: [a, b]
Okay: (3,)
Okay: a[1:4]
Okay: a[:4]
Okay: a[1:]
Okay: a[1:4:2]
E231: ['a','b']
E231: foo(bar,baz)
E231: [{'a':'b'}]

Don't use spaces around the '=' sign in function arguments.

Don't use spaces around the '=' sign when used to indicate a
keyword argument or a default parameter value, except when
using a type annotation.

Okay: def complex(real, imag=0.0):
Okay: return magic(r=real, i=imag)
Okay: boolean(a == b)
Okay: boolean(a != b)
Okay: boolean(a <= b)
Okay: boolean(a >= b)
Okay: def foo(arg: int = 42):
Okay: async def foo(arg: int = 42):

E251: def complex(real, imag = 0.0):
E251: return magic(r = real, i = imag)
E252: def complex(real, image: float=0.0):

Separate inline comments by at least two spaces.

An inline comment is a comment on the same line as a statement.
Inline comments should be separated by at least two spaces from the
statement. They should start with a # and a single space.

Each line of a block comment starts with a # and a single space
(unless it is indented text inside the comment).

Okay: x = x + 1  # Increment x
Okay: x = x + 1    # Increment x
Okay: # Block comment
E261: x = x + 1 # Increment x
E262: x = x + 1  #Increment x
E262: x = x + 1  #  Increment x
E265: #Block comment
E266: ### Block comment

Continuation lines indentation.

Continuation lines should align wrapped elements either vertically
using Python's implicit line joining inside parentheses, brackets
and braces, or using a hanging indent.

When using a hanging indent these considerations should be applied:
- there should be no arguments on the first line, and
- further indentation should be used to clearly distinguish itself
  as a continuation line.

Okay: a = (\n)
E123: a = (\n    )

Okay: a = (\n    42)
E121: a = (\n   42)
E122: a = (\n42)
E123: a = (\n    42\n    )
E124: a = (24,\n     42\n)
E125: if (\n    b):\n    pass
E126: a = (\n        42)
E127: a = (24,\n      42)
E128: a = (24,\n    42)
E129: if (a or\n    b):\n    pass
E131: a = (\n    42\n 24)

Limit all lines to a maximum of 79 characters.

There are still many devices around that are limited to 80 character
lines; plus, limiting windows to 80 characters makes it possible to
have several windows side-by-side.  The default wrapping on such
devices looks ugly.  Therefore, please limit all lines to a maximum
of 79 characters. For flowing long blocks of text (docstrings or
comments), limiting the length to 72 characters is recommended.

Reports error E501.

Continuation lines indentation.

Continuation lines should align wrapped elements either vertically
using Python's implicit line joining inside parentheses, brackets
and braces, or using a hanging indent.

When using a hanging indent these considerations should be applied:
- there should be no arguments on the first line, and
- further indentation should be used to clearly distinguish itself
  as a continuation line.

Okay: a = (\n)
E123: a = (\n    )

Okay: a = (\n    42)
E121: a = (\n   42)
E122: a = (\n42)
E123: a = (\n    42\n    )
E124: a = (24,\n     42\n)
E125: if (\n    b):\n    pass
E126: a = (\n        42)
E127: a = (24,\n      42)
E128: a = (24,\n    42)
E129: if (a or\n    b):\n    pass
E131: a = (\n    42\n 24)

Limit all lines to a maximum of 79 characters.

There are still many devices around that are limited to 80 character
lines; plus, limiting windows to 80 characters makes it possible to
have several windows side-by-side.  The default wrapping on such
devices looks ugly.  Therefore, please limit all lines to a maximum
of 79 characters. For flowing long blocks of text (docstrings or
comments), limiting the length to 72 characters is recommended.

Reports error E501.

Separate inline comments by at least two spaces.

An inline comment is a comment on the same line as a statement.
Inline comments should be separated by at least two spaces from the
statement. They should start with a # and a single space.

Each line of a block comment starts with a # and a single space
(unless it is indented text inside the comment).

Okay: x = x + 1  # Increment x
Okay: x = x + 1    # Increment x
Okay: # Block comment
E261: x = x + 1 # Increment x
E262: x = x + 1  #Increment x
E262: x = x + 1  #  Increment x
E265: #Block comment
E266: ### Block comment

Limit all lines to a maximum of 79 characters.

There are still many devices around that are limited to 80 character
lines; plus, limiting windows to 80 characters makes it possible to
have several windows side-by-side.  The default wrapping on such
devices looks ugly.  Therefore, please limit all lines to a maximum
of 79 characters. For flowing long blocks of text (docstrings or
comments), limiting the length to 72 characters is recommended.

Reports error E501.

Limit all lines to a maximum of 79 characters.

There are still many devices around that are limited to 80 character
lines; plus, limiting windows to 80 characters makes it possible to
have several windows side-by-side.  The default wrapping on such
devices looks ugly.  Therefore, please limit all lines to a maximum
of 79 characters. For flowing long blocks of text (docstrings or
comments), limiting the length to 72 characters is recommended.

Reports error E501.

Each comma, semicolon or colon should be followed by whitespace.

Okay: [a, b]
Okay: (3,)
Okay: a[1:4]
Okay: a[:4]
Okay: a[1:]
Okay: a[1:4:2]
E231: ['a','b']
E231: foo(bar,baz)
E231: [{'a':'b'}]

Limit all lines to a maximum of 79 characters.

There are still many devices around that are limited to 80 character
lines; plus, limiting windows to 80 characters makes it possible to
have several windows side-by-side.  The default wrapping on such
devices looks ugly.  Therefore, please limit all lines to a maximum
of 79 characters. For flowing long blocks of text (docstrings or
comments), limiting the length to 72 characters is recommended.

Reports error E501.

Limit all lines to a maximum of 79 characters.

There are still many devices around that are limited to 80 character
lines; plus, limiting windows to 80 characters makes it possible to
have several windows side-by-side.  The default wrapping on such
devices looks ugly.  Therefore, please limit all lines to a maximum
of 79 characters. For flowing long blocks of text (docstrings or
comments), limiting the length to 72 characters is recommended.

Reports error E501.

Separate inline comments by at least two spaces.

An inline comment is a comment on the same line as a statement.
Inline comments should be separated by at least two spaces from the
statement. They should start with a # and a single space.

Each line of a block comment starts with a # and a single space
(unless it is indented text inside the comment).

Okay: x = x + 1  # Increment x
Okay: x = x + 1    # Increment x
Okay: # Block comment
E261: x = x + 1 # Increment x
E262: x = x + 1  #Increment x
E262: x = x + 1  #  Increment x
E265: #Block comment
E266: ### Block comment