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

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

Cognitive Complexity is a measure of how hard the control flow of a function is to understand. Functions with high Cognitive Complexity will be difficult to maintain.

See

• Cognitive Complexity

We encountered an error attempting to analyze this line.

The print statement was removed in Python 3.0. The built-in function should be used instead.

Noncompliant Code Example

print '1'  # Noncompliant

Compliant Solution

print('1')

The print statement was removed in Python 3.0. The built-in function should be used instead.

Noncompliant Code Example

print '1'  # Noncompliant

Compliant Solution

print('1')

The print statement was removed in Python 3.0. The built-in function should be used instead.

Noncompliant Code Example

print '1'  # Noncompliant

Compliant Solution

print('1')

Merging collapsible if statements increases the code's readability.

Noncompliant Code Example

if condition1:
    if condition2:
        # ...

Compliant Solution

if condition1 and condition2:
    # ...

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.

Trailing whitespace is superfluous.

The warning returned varies on whether the line itself is blank,
for easier filtering for those who want to indent their blank lines.

Okay: spam(1)\n#
W291: spam(1) \n#
W293: class Foo(object):\n    \n    bang = 12

Surround operators with a single space on either side.

- Always surround these binary operators with a single space on
  either side: assignment (=), augmented assignment (+=, -= etc.),
  comparisons (==, <, >, !=, <=, >=, in, not in, is, is not),
  Booleans (and, or, not).

- If operators with different priorities are used, consider adding
  whitespace around the operators with the lowest priorities.

Okay: i = i + 1
Okay: submitted += 1
Okay: x = x * 2 - 1
Okay: hypot2 = x * x + y * y
Okay: c = (a + b) * (a - b)
Okay: foo(bar, key='word', *args, **kwargs)
Okay: alpha[:-i]

E225: i=i+1
E225: submitted +=1
E225: x = x /2 - 1
E225: z = x **y
E225: z = 1and 1
E226: c = (a+b) * (a-b)
E226: hypot2 = x*x + y*y
E227: c = a|b
E228: msg = fmt%(errno, errmsg)

Avoid extraneous whitespace.

Avoid extraneous whitespace in these situations:
- Immediately inside parentheses, brackets or braces.
- Immediately before a comma, semicolon, or colon.

Okay: spam(ham[1], {eggs: 2})
E201: spam( ham[1], {eggs: 2})
E201: spam(ham[ 1], {eggs: 2})
E201: spam(ham[1], { eggs: 2})
E202: spam(ham[1], {eggs: 2} )
E202: spam(ham[1 ], {eggs: 2})
E202: spam(ham[1], {eggs: 2 })

E203: if x == 4: print x, y; x, y = y , x
E203: if x == 4: print x, y ; x, y = y, x
E203: if x == 4 : print x, y; x, y = y, x

Separate top-level function and class definitions with two blank lines.

Method definitions inside a class are separated by a single blank
line.

Extra blank lines may be used (sparingly) to separate groups of
related functions.  Blank lines may be omitted between a bunch of
related one-liners (e.g. a set of dummy implementations).

Use blank lines in functions, sparingly, to indicate logical
sections.

Okay: def a():\n    pass\n\n\ndef b():\n    pass
Okay: def a():\n    pass\n\n\nasync def b():\n    pass
Okay: def a():\n    pass\n\n\n# Foo\n# Bar\n\ndef b():\n    pass
Okay: default = 1\nfoo = 1
Okay: classify = 1\nfoo = 1

E301: class Foo:\n    b = 0\n    def bar():\n        pass
E302: def a():\n    pass\n\ndef b(n):\n    pass
E302: def a():\n    pass\n\nasync def b(n):\n    pass
E303: def a():\n    pass\n\n\n\ndef b(n):\n    pass
E303: def a():\n\n\n\n    pass
E304: @decorator\n\ndef a():\n    pass
E305: def a():\n    pass\na()
E306: def a():\n    def b():\n        pass\n    def c():\n        pass

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.

Never use the characters 'l', 'O', or 'I' as variable names.

In some fonts, these characters are indistinguishable from the
numerals one and zero. When tempted to use 'l', use 'L' instead.

Okay: L = 0
Okay: o = 123
Okay: i = 42
E741: l = 0
E741: O = 123
E741: I = 42

Variables can be bound in several other contexts, including class
and function definitions, 'global' and 'nonlocal' statements,
exception handlers, and 'with' and 'for' statements.
In addition, we have a special handling for function parameters.

Okay: except AttributeError as o:
Okay: with lock as L:
Okay: foo(l=12)
Okay: for a in foo(l=12):
E741: except AttributeError as O:
E741: with lock as l:
E741: global I
E741: nonlocal l
E741: def foo(l):
E741: def foo(l=12):
E741: l = foo(l=12)
E741: for l in range(10):
E742: class I(object):
E743: def l(x):

Surround operators with a single space on either side.

- Always surround these binary operators with a single space on
  either side: assignment (=), augmented assignment (+=, -= etc.),
  comparisons (==, <, >, !=, <=, >=, in, not in, is, is not),
  Booleans (and, or, not).

- If operators with different priorities are used, consider adding
  whitespace around the operators with the lowest priorities.

Okay: i = i + 1
Okay: submitted += 1
Okay: x = x * 2 - 1
Okay: hypot2 = x * x + y * y
Okay: c = (a + b) * (a - b)
Okay: foo(bar, key='word', *args, **kwargs)
Okay: alpha[:-i]

E225: i=i+1
E225: submitted +=1
E225: x = x /2 - 1
E225: z = x **y
E225: z = 1and 1
E226: c = (a+b) * (a-b)
E226: hypot2 = x*x + y*y
E227: c = a|b
E228: msg = fmt%(errno, errmsg)

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.

Surround operators with a single space on either side.

- Always surround these binary operators with a single space on
  either side: assignment (=), augmented assignment (+=, -= etc.),
  comparisons (==, <, >, !=, <=, >=, in, not in, is, is not),
  Booleans (and, or, not).

- If operators with different priorities are used, consider adding
  whitespace around the operators with the lowest priorities.

Okay: i = i + 1
Okay: submitted += 1
Okay: x = x * 2 - 1
Okay: hypot2 = x * x + y * y
Okay: c = (a + b) * (a - b)
Okay: foo(bar, key='word', *args, **kwargs)
Okay: alpha[:-i]

E225: i=i+1
E225: submitted +=1
E225: x = x /2 - 1
E225: z = x **y
E225: z = 1and 1
E226: c = (a+b) * (a-b)
E226: hypot2 = x*x + y*y
E227: c = a|b
E228: msg = fmt%(errno, errmsg)

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.

Trailing whitespace is superfluous.

The warning returned varies on whether the line itself is blank,
for easier filtering for those who want to indent their blank lines.

Okay: spam(1)\n#
W291: spam(1) \n#
W293: class Foo(object):\n    \n    bang = 12

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

Never use the characters 'l', 'O', or 'I' as variable names.

In some fonts, these characters are indistinguishable from the
numerals one and zero. When tempted to use 'l', use 'L' instead.

Okay: L = 0
Okay: o = 123
Okay: i = 42
E741: l = 0
E741: O = 123
E741: I = 42

Variables can be bound in several other contexts, including class
and function definitions, 'global' and 'nonlocal' statements,
exception handlers, and 'with' and 'for' statements.
In addition, we have a special handling for function parameters.

Okay: except AttributeError as o:
Okay: with lock as L:
Okay: foo(l=12)
Okay: for a in foo(l=12):
E741: except AttributeError as O:
E741: with lock as l:
E741: global I
E741: nonlocal l
E741: def foo(l):
E741: def foo(l=12):
E741: l = foo(l=12)
E741: for l in range(10):
E742: class I(object):
E743: def l(x):

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

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.

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

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.

When catching exceptions, mention specific exceptions when possible.

Okay: except Exception:
Okay: except BaseException:
E722: except:

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.

Never use the characters 'l', 'O', or 'I' as variable names.

In some fonts, these characters are indistinguishable from the
numerals one and zero. When tempted to use 'l', use 'L' instead.

Okay: L = 0
Okay: o = 123
Okay: i = 42
E741: l = 0
E741: O = 123
E741: I = 42

Variables can be bound in several other contexts, including class
and function definitions, 'global' and 'nonlocal' statements,
exception handlers, and 'with' and 'for' statements.
In addition, we have a special handling for function parameters.

Okay: except AttributeError as o:
Okay: with lock as L:
Okay: foo(l=12)
Okay: for a in foo(l=12):
E741: except AttributeError as O:
E741: with lock as l:
E741: global I
E741: nonlocal l
E741: def foo(l):
E741: def foo(l=12):
E741: l = foo(l=12)
E741: for l in range(10):
E742: class I(object):
E743: def l(x):

Never use the characters 'l', 'O', or 'I' as variable names.

In some fonts, these characters are indistinguishable from the
numerals one and zero. When tempted to use 'l', use 'L' instead.

Okay: L = 0
Okay: o = 123
Okay: i = 42
E741: l = 0
E741: O = 123
E741: I = 42

Variables can be bound in several other contexts, including class
and function definitions, 'global' and 'nonlocal' statements,
exception handlers, and 'with' and 'for' statements.
In addition, we have a special handling for function parameters.

Okay: except AttributeError as o:
Okay: with lock as L:
Okay: foo(l=12)
Okay: for a in foo(l=12):
E741: except AttributeError as O:
E741: with lock as l:
E741: global I
E741: nonlocal l
E741: def foo(l):
E741: def foo(l=12):
E741: l = foo(l=12)
E741: for l in range(10):
E742: class I(object):
E743: def l(x):

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.

Negative comparison should be done using "not in" and "is not".

Okay: if x not in y:\n    pass
Okay: assert (X in Y or X is Z)
Okay: if not (X in Y):\n    pass
Okay: zz = x is not y
E713: Z = not X in Y
E713: if not X.B in Y:\n    pass
E714: if not X is Y:\n    pass
E714: Z = not X.B is Y

Never use the characters 'l', 'O', or 'I' as variable names.

In some fonts, these characters are indistinguishable from the
numerals one and zero. When tempted to use 'l', use 'L' instead.

Okay: L = 0
Okay: o = 123
Okay: i = 42
E741: l = 0
E741: O = 123
E741: I = 42

Variables can be bound in several other contexts, including class
and function definitions, 'global' and 'nonlocal' statements,
exception handlers, and 'with' and 'for' statements.
In addition, we have a special handling for function parameters.

Okay: except AttributeError as o:
Okay: with lock as L:
Okay: foo(l=12)
Okay: for a in foo(l=12):
E741: except AttributeError as O:
E741: with lock as l:
E741: global I
E741: nonlocal l
E741: def foo(l):
E741: def foo(l=12):
E741: l = foo(l=12)
E741: for l in range(10):
E742: class I(object):
E743: def l(x):

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.

Never use the characters 'l', 'O', or 'I' as variable names.

In some fonts, these characters are indistinguishable from the
numerals one and zero. When tempted to use 'l', use 'L' instead.

Okay: L = 0
Okay: o = 123
Okay: i = 42
E741: l = 0
E741: O = 123
E741: I = 42

Variables can be bound in several other contexts, including class
and function definitions, 'global' and 'nonlocal' statements,
exception handlers, and 'with' and 'for' statements.
In addition, we have a special handling for function parameters.

Okay: except AttributeError as o:
Okay: with lock as L:
Okay: foo(l=12)
Okay: for a in foo(l=12):
E741: except AttributeError as O:
E741: with lock as l:
E741: global I
E741: nonlocal l
E741: def foo(l):
E741: def foo(l=12):
E741: l = foo(l=12)
E741: for l in range(10):
E742: class I(object):
E743: def l(x):

Separate top-level function and class definitions with two blank lines.

Method definitions inside a class are separated by a single blank
line.

Extra blank lines may be used (sparingly) to separate groups of
related functions.  Blank lines may be omitted between a bunch of
related one-liners (e.g. a set of dummy implementations).

Use blank lines in functions, sparingly, to indicate logical
sections.

Okay: def a():\n    pass\n\n\ndef b():\n    pass
Okay: def a():\n    pass\n\n\nasync def b():\n    pass
Okay: def a():\n    pass\n\n\n# Foo\n# Bar\n\ndef b():\n    pass
Okay: default = 1\nfoo = 1
Okay: classify = 1\nfoo = 1

E301: class Foo:\n    b = 0\n    def bar():\n        pass
E302: def a():\n    pass\n\ndef b(n):\n    pass
E302: def a():\n    pass\n\nasync def b(n):\n    pass
E303: def a():\n    pass\n\n\n\ndef b(n):\n    pass
E303: def a():\n\n\n\n    pass
E304: @decorator\n\ndef a():\n    pass
E305: def a():\n    pass\na()
E306: def a():\n    def b():\n        pass\n    def c():\n        pass

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.

Never use the characters 'l', 'O', or 'I' as variable names.

In some fonts, these characters are indistinguishable from the
numerals one and zero. When tempted to use 'l', use 'L' instead.

Okay: L = 0
Okay: o = 123
Okay: i = 42
E741: l = 0
E741: O = 123
E741: I = 42

Variables can be bound in several other contexts, including class
and function definitions, 'global' and 'nonlocal' statements,
exception handlers, and 'with' and 'for' statements.
In addition, we have a special handling for function parameters.

Okay: except AttributeError as o:
Okay: with lock as L:
Okay: foo(l=12)
Okay: for a in foo(l=12):
E741: except AttributeError as O:
E741: with lock as l:
E741: global I
E741: nonlocal l
E741: def foo(l):
E741: def foo(l=12):
E741: l = foo(l=12)
E741: for l in range(10):
E742: class I(object):
E743: def l(x):