Best practices

These checks ensure that you follow the best practices.

The source for these best practices is hidden inside countless hours we have spent debugging software or reviewing it.

How do we find inspiration for new rules? We find some ugly code during code reviews and audits. Then we forbid to use this bad code forever. So, this error will never return to our codebase.

Summary

WrongMagicCommentViolation	Restricts to use several control (or magic) comments.
WrongDocCommentViolation	Forbids to use empty doc comments (#:).
OveruseOfNoqaCommentViolation	Forbids to use too many # noqa comments.
OveruseOfNoCoverCommentViolation	Forbids to use too many # pragma: no cover comments.
ComplexDefaultValueViolation	Forbids to use complex defaults.
LoopVariableDefinitionViolation	Forbids to use anything rather than ast.Name to define loop variables.
ContextManagerVariableDefinitionViolation	Forbids to use anything rather than ast.Name to define contexts.
MutableModuleConstantViolation	Forbids mutable constants on a module level.
SameElementsInConditionViolation	Forbids to use the same logical conditions in one expression.
HeterogenousCompareViolation	Forbids to heterogenous operators in one compare.
WrongModuleMetadataViolation	Forbids to have some module level variables.
EmptyModuleViolation	Forbids to have empty modules.
InitModuleHasLogicViolation	Forbids to have logic inside __init__ module.
BadMagicModuleFunctionViolation	Forbids to use __getaddr__ and __dir__ module magic methods.
WrongUnpackingViolation	Forbids to have tuple unpacking with side-effects.
DuplicateExceptionViolation	Forbids to have the same exception class in multiple except blocks.
YieldInComprehensionViolation	Forbids to have yield keyword inside comprehensions.
NonUniqueItemsInHashViolation	Forbids to have duplicate items in hashes.
BaseExceptionSubclassViolation	Forbids to have duplicate items in set literals.
TryExceptMultipleReturnPathViolation	Forbids to use multiple returning paths with try / except case.
WrongKeywordViolation	Forbids to use some python keywords.
WrongFunctionCallViolation	Forbids to call some built-in functions.
FutureImportViolation	Forbids to use __future__ imports.
RaiseNotImplementedViolation	Forbids to use NotImplemented error.
BaseExceptionViolation	Forbids to use BaseException exception.
BooleanPositionalArgumentViolation	Forbids to pass booleans as non-keyword parameters.
LambdaInsideLoopViolation	Forbids to use lambda inside loops.
UnreachableCodeViolation	Forbids to have unreachable code.
StatementHasNoEffectViolation	Forbids to have statements that do nothing.
MultipleAssignmentsViolation	Forbids to have multiple assignments on the same line.
NestedFunctionViolation	Forbids to have nested functions.
NestedClassViolation	Forbids to use nested classes.
MagicNumberViolation	Forbids to use magic numbers in your code.
NestedImportViolation	Forbids to have nested imports in functions.
ReassigningVariableToItselfViolation	Forbids to assign variable to itself.
ListMultiplyViolation	Forbids to multiply lists.
ProtectedModuleViolation	Forbids to import protected modules.
ProtectedAttributeViolation	Forbids to use protected attributes and methods.
StopIterationInsideGeneratorViolation	Forbids to raise StopIteration inside generators.
WrongUnicodeEscapeViolation	Forbids to use unicode escape sequences in binary strings.
BlockAndLocalOverlapViolation	Forbids to local and block variables to overlap.
ControlVarUsedAfterBlockViolation	Forbids to use control variables after the block body.
OuterScopeShadowingViolation	Forbids to shadow variables from outer scopes.
UnhashableTypeInHashViolation	Forbids to use exlicit unhashable types as set items and dict keys.
WrongKeywordConditionViolation	Forbids to use exlicit falsly-evaluated conditions with several keywords.

Best practices

class WrongMagicCommentViolation(node=None, text=None)

Bases: wemake_python_styleguide.violations.base.SimpleViolation

Restricts to use several control (or magic) comments.

We do not allow to use:

1. # noqa comment without specified violations

2. # type: some_type comments to specify a type for typed_ast

This violation is reported at the top of the module, so it cannot be locally ignored.

Reasoning:

We cover several different use-cases in a single rule. # noqa comment is restricted because it can hide other violations. # type: some_type comment is restricted because we can already use type annotations instead.

Solution:

Use # noqa comments with specified error types. Use type annotations to specify types.

We still allow to use # type: ignore comment. Since sometimes it is totally required.

Example:

# Correct:
type = MyClass.get_type()  # noqa: A001
coordinate: int = 10
some.int_field = 'text'  # type: ignore

number: int
for number in some_untyped_iterable():
    ...

# Wrong:
type = MyClass.get_type()  # noqa
coordinate = 10  # type: int

New in version 0.1.0.

code = 400

error_template = 'Found wrong magic comment: {0}'

class WrongDocCommentViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.TokenizeViolation

Forbids to use empty doc comments (#:).

Reasoning:

Doc comments are used to provide a documentation. But supplying empty doc comments breaks this use-case. It is unclear why they can be used with no contents.

Solution:

Add some documentation to this comment. Or remove it.

Empty doc comments are not caught by the default pycodestyle checks.

Example:

# Correct:
#: List of allowed names:
NAMES_WHITELIST = ['feature', 'bug', 'research']

# Wrong:
#:
NAMES_WHITELIST = ['feature', 'bug', 'research']

New in version 0.1.0.

code = 401

error_template = 'Found wrong doc comment'

class OveruseOfNoqaCommentViolation(node=None, text=None)

Bases: wemake_python_styleguide.violations.base.SimpleViolation

Forbids to use too many # noqa comments.

We count it on a per-module basis. We use 10 as a hard limit.

Reasoning:

Having too many # noqa comments make your code less readable and clearly indicates that there’s something wrong with it.

Solution:

Refactor your code to match our style. Or use a config file to switch off some checks.

New in version 0.7.0.

error_template = 'Found `noqa` comments overuse: {0}'

code = 402

class OveruseOfNoCoverCommentViolation(node=None, text=None)

Bases: wemake_python_styleguide.violations.base.SimpleViolation

Forbids to use too many # pragma: no cover comments.

We count it on a per-module basis. We use 5 as a default value.

Reasoning:

Having too many # pragma: no cover comments clearly indicates that there’s something wrong with it. Moreover, it makes your tests useless, since they do not cover a big partion of your code.

Solution:

Refactor your code to much the style. Or use a config file to switch off some checks.

New in version 0.8.0.

error_template = 'Found `noqa` comments overuse: {0}'

code = 403

class ComplexDefaultValueViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use complex defaults.

Anything that is not a ast.Name, ast.Attribute, ast.Str, ast.NameConstant, ast.Tuple, ast.Bytes, ast.Num or ast.Ellipsis should be moved out from defaults.

Reasoning:

It can be tricky. Nothing stops you from making database calls or http requests in such expressions. It is also not readable for us.

Solution:

Move the expression out from default value.

Example:

# Correct:
SHOULD_USE_DOCTEST = 'PYFLAKES_DOCTEST' in os.environ
def __init__(self, with_doctest=SHOULD_USE_DOCTEST):

# Wrong:
def __init__(self, with_doctest='PYFLAKES_DOCTEST' in os.environ):

New in version 0.8.0.

Changed in version 0.11.0.

error_template = 'Found complex default value'

code = 404

previous_codes = {459}

class LoopVariableDefinitionViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use anything rather than ast.Name to define loop variables.

Reasoning:

When defining a for loop with attributes, indexes, calls, or any other nodes it does dirty things inside.

Solution:

Use regular ast.Name variables. Or tuple of ast.Name variables. Star names are also fine.

Example:

# Correct:
for person in database.people():
    ...

# Wrong:
for context['person'] in database.people():
    ...

New in version 0.8.0.

Changed in version 0.11.0.

error_template = 'Found wrong `for` loop variable definition'

code = 405

previous_codes = {460}

class ContextManagerVariableDefinitionViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use anything rather than ast.Name to define contexts.

Reasoning:

When defining a with context managers with attributes, indexes, calls, or any other nodes it does dirty things inside.

Solution:

Use regular ast.Name variables. Or tuple of ast.Name variables. Star names are also fine.

Example:

# Correct:
with open('README.md') as readme:
    ...

# Wrong:
with open('README.md') as files['readme']:
    ...

New in version 0.8.0.

Changed in version 0.11.0.

error_template = 'Found wrong context manager variable definition'

code = 406

previous_codes = {461}

class MutableModuleConstantViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids mutable constants on a module level.

Reasoning:

Constants should be immutable.

Solution:

Use immutable types for constants.

We only treat ast.Set, ast.Dict, ast.List, and comprehensions as mutable things. All other nodes are still fine.

Example:

# Correct:
import types
CONST1 = frozenset((1, 2, 3))
CONST2 = (1, 2, 3)
CONST3 = types.MappingProxyType({'key': 'value'})

# Wrong:
CONST1 = {1, 2, 3}
CONST2 = [x for x in some()]
CONST3 = {'key': 'value'}

New in version 0.10.0.

Changed in version 0.11.0.

error_template = 'Found mutable module constant'

code = 407

previous_codes = {466}

class SameElementsInConditionViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use the same logical conditions in one expression.

Reasoning:

Using the same name in logical condition more that once indicates that you are either making a logical mistake, or just over-complicating your design.

Solution:

Remove the duplicating condition.

Example:

# Correct:
if some_value or other_value:
    ...

# Wrong:
if some_value or some_value:
    ...

New in version 0.10.0.

Changed in version 0.11.0.

error_template = 'Found duplicate logical condition'

code = 408

previous_codes = {469}

class HeterogenousCompareViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to heterogenous operators in one compare.

Note, that we allow to mix > with >= and < with <= operators.

Reasoning:

This is hard to read and understand.

Solution:

Refactor the expression to have separate parts joined with and boolean operator.

Example:

# Correct:
if x == y == z:
    ...

if x > y >= z:
    ...

# Wrong:
if x > y == 5:
    ...

if x == y != z:
    ...

New in version 0.10.0.

Changed in version 0.11.0.

error_template = 'Found heterogenous compare'

code = 409

previous_codes = {471}

class WrongModuleMetadataViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have some module level variables.

Reasoning:

We discourage using module variables like __author__, because code should not contain any metadata.

Solution:

Place all the metadata in setup.py, setup.cfg, or pyproject.toml. Use proper docstrings and packaging classifiers. Use pkg_resources if you need to import this data into your app.

See MODULE_METADATA_VARIABLES_BLACKLIST for full list of bad names.

Example:

# Wrong:
__author__ = 'Nikita Sobolev'
__version__ = 0.1.2

New in version 0.1.0.

error_template = 'Found wrong metadata variable: {0}'

code = 410

class EmptyModuleViolation(node=None, text=None)

Bases: wemake_python_styleguide.violations.base.SimpleViolation

Forbids to have empty modules.

Reasoning:

Why is it even there? Do not pollute your project with empty files.

Solution:

If you have an empty module there are two ways to handle that:

1. delete it

2. drop some documentation in it, so you will explain why it is there

New in version 0.1.0.

error_template = 'Found empty module'

code = 411

class InitModuleHasLogicViolation(node=None, text=None)

Bases: wemake_python_styleguide.violations.base.SimpleViolation

Forbids to have logic inside __init__ module.

Reasoning:

If you have logic inside the __init__ module it means several things:

1. you are keeping some outdated stuff there, you need to refactor

2. you are placing this logic into the wrong file, just create another one

3. you are doing some dark magic, and you should not do that

Solution:

Put your code in other modules.

However, we allow to have some contents inside the __init__ module:

1. comments, since they are dropped before AST comes in play

2. docs string, because sometimes it is required to state something

It is also fine when you have different users that use your code. And you do not want to break everything for them. In this case this rule can be configured.

Configuration:

This rule is configurable with --i-control-code. Default: True

New in version 0.1.0.

error_template = 'Found `__init__.py` module with logic'

code = 412

class BadMagicModuleFunctionViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use __getaddr__ and __dir__ module magic methods.

Reasoning:

It does not bring any features, only making it harder to understand what is going on.

Solution:

Refactor your code to use custom methods instead.

Configuration:

This rule is configurable with --i-control-code. Default: True

New in version 0.9.0.

error_template = 'Found bad magic module function: {0}'

code = 413

class WrongUnpackingViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have tuple unpacking with side-effects.

Reasoning:

Having unpacking with side-effects is very dirty. You might get in serious and very hard-to-debug troubles because of this technique. So, do not use it.

Solution:

Use unpacking with only variables, not any other entities.

Example:

# Correct:
first, second = some()

# Wrong:
first, some_dict['alias'] = some()

New in version 0.6.0.

Changed in version 0.11.0.

error_template = 'Found incorrect unpacking target'

code = 414

previous_codes = {446}

class DuplicateExceptionViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have the same exception class in multiple except blocks.

Reasoning:

Having the same exception name in different blocks means that something is not right: since only one branch will work. Other one will always be ignored. So, that is clearly an error.

Solution:

Use unique exception handling rules.

Example:

# Correct:
try:
    ...
except ValueError:
    ...

# Wrong:
try:
    ...
except ValueError:
    ...
except ValueError:
    ...

New in version 0.6.0.

Changed in version 0.11.0.

error_template = 'Found duplicate exception: {0}'

code = 415

previous_codes = {447}

class YieldInComprehensionViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have yield keyword inside comprehensions.

Reasoning:

Having the yield keyword inside comprehensions is error-prone. You can shoot yourself in a foot by an inaccurate usage of this feature.

Solution:

Use regular for loops with yield keywords. Or create a separate generator function.

Example:

# Wrong:
list((yield letter) for letter in 'ab')
# Will resilt in: ['a', None, 'b', None]

list([(yield letter) for letter in 'ab'])
# Will result in: ['a', 'b']

See also

https://github.com/satwikkansal/wtfPython#-yielding-none

New in version 0.7.0.

Changed in version 0.11.0.

error_template = 'Found `yield` inside comprehension'

code = 416

previous_codes = {448}

class NonUniqueItemsInHashViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have duplicate items in hashes.

Reasoning:

When you explicitly put duplicate items in set literals or in dict keys it just does not make any sense. Since hashes cannot contain duplicate items and they will be removed anyway.

Solution:

Remove duplicate items.

Example:

# Correct:
some_set = {'a', variable1}
some_set = {make_call(), make_call()}

# Wrong:
some_set = {'a', 'a', variable1, variable1}

Things that we consider duplicates: builtins and variables. These nodes are not checked because they may return different results:

• function and method calls

• comprehensions

• attributes

• subscribe operations

New in version 0.7.0.

Changed in version 0.11.0.

Changed in version 0.12.0.

error_template = 'Found non-unique item in hash: {0}'

code = 417

previous_codes = {449}

class BaseExceptionSubclassViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have duplicate items in set literals.

Reasoning:

BaseException is a special case: it is not designed to be extended by users. A lot of your except Exception cases won’t work. That’s incorrect and dangerous.

Solution:

Change the base class to Exception.

Example:

# Correct:
class MyException(Exception):
    ...

# Wrong:
class MyException(BaseException):
    ...

See also

https://docs.python.org/3/library/exceptions.html#exception-hierarchy

New in version 0.7.0.

Changed in version 0.11.0.

error_template = 'Found exception inherited from `BaseException`'

code = 418

previous_codes = {450}

class TryExceptMultipleReturnPathViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use multiple returning paths with try / except case.

Note, that we check for any return, break, or raise nodes.

Reasoning:

The problem with return in else and finally is that it is impossible to say what value is going to be actually returned without looking up the implementation details. Why? Because return does not expect that some other code will be executed after it. But, finally is always executed, even after return. And else will not be executed when there are no exceptions in try case and a return statement.

Solution:

Remove return from one of the cases.

Example:

# Correct:
try:
    return 1
except YourException:
    ...
finally:
    clear_things_up()

# Wrong:
try:
    return 1  # this line will never return
except Exception:
    ...
finally:
    return 2  # this line will actually return

try:
    return 1  # this line will actually return
except ZeroDivisionError:
    ...
else:
    return 0  # this line will never return

New in version 0.7.0.

Changed in version 0.11.0.

Changed in version 0.12.0.

error_template = 'Found `try`/`else`/`finally` with multiple return paths'

code = 419

previous_codes = {458}

class WrongKeywordViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use some python keywords.

Reasoning:

Using some keywords generally gives you more pain that relieve.

del keyword is not composable with other functions, you cannot pass it as a regular function. It is also quite error-prone due to __del__ magic method complexity and that del is actually used to nullify variables and delete them from the execution scope. Moreover, it has a lot of substitutions. You won’t miss it!

pass keyword is just useless by design. There’s no usecase for it. Because it does literally nothing.

global and nonlocal promote bad-practices of having an external mutable state somewhere. This solution does not scale. And leads to multiple possible mistakes in the future.

Solution:

Solutions differ from keyword to keyword. pass should be replaced with docstring or contextlib.suppress. del should be replaced with specialized methods like .pop(). global and nonlocal usages should be refactored.

New in version 0.1.0.

error_template = 'Found wrong keyword: {0}'

code = 420

class WrongFunctionCallViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to call some built-in functions.

Reasoning:

Some functions are only suitable for very specific use cases, we forbid to use them in a free manner.

See FUNCTIONS_BLACKLIST for the full list of blacklisted functions.

See also

https://www.youtube.com/watch?v=YjHsOrOOSuI

New in version 0.1.0.

error_template = 'Found wrong function call: {0}'

code = 421

class FutureImportViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use __future__ imports.

Reasoning:

Almost all __future__ imports are legacy python2 compatibility tools that are no longer required.

Solution:

Remove them. Drop python2 support.

Except, there are some new ones for python4 support. See FUTURE_IMPORTS_WHITELIST for the full list of allowed future imports.

Example:

# Correct:
from __future__ import annotations

# Wrong:
from __future__ import print_function

New in version 0.1.0.

error_template = 'Found future import: {0}'

code = 422

class RaiseNotImplementedViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use NotImplemented error.

Reasoning:

These two violations look so similar. But, these violations have different use cases. Use cases of NotImplemented is too limited to be generally available.

Solution:

Use NotImplementedError.

Example:

# Correct:
raise NotImplementedError('To be done')

# Wrong:
raise NotImplemented

New in version 0.1.0.

See also

https://stackoverflow.com/a/44575926/4842742

error_template = 'Found raise NotImplemented'

code = 423

class BaseExceptionViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use BaseException exception.

Reasoning:

We can silence system exit and keyboard interrupt with this exception handler. It is almost the same as raw except: block.

Solution:

Handle Exception, KeyboardInterrupt, GeneratorExit, and SystemExit separately. Do not use the plain except: keyword.

Example:

# Correct:
except Exception as ex: ...

# Wrong:
except BaseException as ex: ...

New in version 0.3.0.

See also

https://docs.python.org/3/library/exceptions.html#exception-hierarchy https://help.semmle.com/wiki/pages/viewpage.action?pageId=1608527

error_template = 'Found except `BaseException`'

code = 424

class BooleanPositionalArgumentViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to pass booleans as non-keyword parameters.

Reasoning:

Passing boolean as regular positional parameters is very non-descriptive. It is almost impossible to tell, what does this parameter means. And you almost always have to look up the implementation to tell what is going on.

Solution:

Pass booleans as keywords only. This will help you to save extra context on what’s going on.

Example:

# Correct:
UsersRepository.update(cache=True)

# Wrong:
UsersRepository.update(True)

New in version 0.6.0.

error_template = 'Found boolean non-keyword argument: {0}'

code = 425

class LambdaInsideLoopViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use lambda inside loops.

Reasoning:

It is error-prone to use lambda inside for and while loops due to the famous late-binding.

Solution:

Use regular functions, factory functions, or partial functions. Save yourself from possible confusion.

Example:

# Correct:
for index in range(10):
    some.append(partial_function(index))

# Wrong:
for index in range(10):
    some.append(lambda index=index: index * 10))
    other.append(lambda: index * 10))

New in version 0.5.0.

Changed in version 0.11.0.

See also

https://docs.python-guide.org/writing/gotchas/#late-binding-closures

error_template = "Found `lambda` in loop's body"

code = 426

previous_codes = {442}

class UnreachableCodeViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have unreachable code.

What is unreachable code? It is some lines of code that cannot be executed by python’s interpreter.

This is probably caused by return or raise statements. However, we can not cover 100% of truly unreachable code by this rule. This happens due to the dynamic nature of python. For example, detecting that 1 / some_value would sometimes raise an exception is too complicated and is out of the scope of this rule.

Reasoning:

Having dead code in your project is an indicator that you do not care about your code base at all. It dramatically reduces code quality and readability. It also demotivates team members.

Solution:

Delete any unreachable code you have. Or refactor it, if this happens by your mistake.

Example:

# Correct:
def some_function():
    print('This line is reachable, all good')
    return 5

# Wrong:
def some_function():
    return 5
    print('This line is unreachable')

New in version 0.5.0.

Changed in version 0.11.0.

error_template = 'Found unreachable code'

code = 427

previous_codes = {443}

class StatementHasNoEffectViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have statements that do nothing.

Reasoning:

Statements that just access the value or expressions used as statements indicate that your code contains deadlines. They just pollute your codebase and do nothing.

Solution:

Refactor your code in case it was a typo or error. Or just delete this code.

Example:

# Correct:
def some_function():
    price = 8 + 2
    return price

# Wrong:
def some_function():
    8 + 2
    print

New in version 0.5.0.

Changed in version 0.11.0.

error_template = 'Found statement that has no effect'

code = 428

previous_codes = {444}

class MultipleAssignmentsViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have multiple assignments on the same line.

Reasoning:

Multiple assignments on the same line might not do what you think they do. They can also grown pretty long. And you will not notice the rising complexity of your code.

Solution:

Use separate lines for each assignment.

Example:

# Correct:
a = 1
b = 1

# Wrong:
a = b = 1

New in version 0.6.0.

Changed in version 0.11.0.

error_template = 'Found multiple assign targets'

code = 429

previous_codes = {445}

class NestedFunctionViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have nested functions.

Reasoning:

Nesting functions is a bad practice. It is hard to test them, it is hard then to separate them. People tend to overuse closures, so it’s hard to manage the dataflow.

Solution:

Just write flat functions, there’s no need to nest them. Pass parameters as normal arguments, do not use closures. Until you need them for decorators or factories.

We also disallow to nest lambda and async functions.

See NESTED_FUNCTIONS_WHITELIST for the whole list of whitelisted names.

Example:

# Correct:
def do_some(): ...
def other(): ...

# Wrong:
def do_some():
    def inner():
        ...

New in version 0.1.0.

error_template = 'Found nested function: {0}'

code = 430

class NestedClassViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use nested classes.

Reasoning:

Nested classes are really hard to manage. You can not even create an instance of this class in many cases. Testing them is also really hard.

Solution:

Just write flat classes, there’s no need nest them. If you are nesting classes inside a function for parametrization, then you will probably need to use different design (or metaclasses).

See NESTED_CLASSES_WHITELIST for the full list of whitelisted names.

Example:

# Correct:
class Some(object): ...
class Other(object): ...

# Wrong:
class Some(object):
    class Inner(object):
        ...

New in version 0.1.0.

error_template = 'Found nested class: {0}'

code = 431

class MagicNumberViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use magic numbers in your code.

What we call a “magic number”? Well, it is actually any number that appears in your code out of nowhere. Like 42. Or 0.32.

Reasoning:

It is very hard to remember what these numbers actually mean. Why were they used? Should they ever be changed? Or are they eternal like 3.14?

Solution:

Give these numbers a name! Move them to a separate variable, giving more context to the reader. And by moving things into new variables you will trigger other complexity checks.

Example:

# Correct:
price_in_euro = 3.33  # could be changed later
total = get_items_from_cart() * price_in_euro

# Wrong:
total = get_items_from_cart() * 3.33

What are numbers that we exclude from this check? Any numbers that are assigned to a variable, array, dictionary, or keyword arguments inside a function. int numbers that are in range [-10, 10] and some other common numbers, that are defined in MAGIC_NUMBERS_WHITELIST

New in version 0.1.0.

See also

https://en.wikipedia.org/wiki/Magic_number_(programming)

code = 432

error_template = 'Found magic number: {0}'

class NestedImportViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have nested imports in functions.

Reasoning:

Usually, nested imports are used to fix the import cycle. So, nested imports show that there’s an issue with your design.

Solution:

You don’t need nested imports, you need to refactor your code. Introduce a new module or find another way to do what you want to do. Rethink how your layered architecture should look like.

Example:

# Correct:
from my_module import some_function

def some(): ...

# Wrong:
def some():
    from my_module import some_function

New in version 0.1.0.

Changed in version 0.11.0.

See also

https://github.com/seddonym/layer_linter

error_template = 'Found nested import'

code = 433

previous_codes = {435}

class ReassigningVariableToItselfViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to assign variable to itself.

Reasoning:

There is no need to do that. Generally, it is an indication of some errors or just dead code.

Example:

# Correct:
some = some + 1
x_coord, y_coord = y_coord, x_coord

# Wrong:
some = some
x_coord, y_coord = x_coord, y_coord

New in version 0.3.0.

Changed in version 0.11.0.

error_template = 'Found reassigning variable to itself: {0}'

code = 434

previous_codes = {438}

class ListMultiplyViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to multiply lists.

Reasoning:

When you multiply lists - it does not create new values, it creates references to the existing value. It is not what people mean in 99.9% of cases.

Solution:

Use list comprehension or loop instead.

Example:

# Wrong:
my_list = [1, 2, 3] * 3

See also

https://github.com/satwikkansal/wtfPython#-explanation-8

New in version 0.12.0.

error_template = 'Found list multiply'

code = 435

class ProtectedModuleViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to import protected modules.

Reasoning:

When importing protected modules we break a contract that authors of this module enforce. This way we are not respecting encapsulation and it may break our code at any moment.

Solution:

Do not import anything from protected modules. Respect the encapsulation.

Example:

# Correct:
from some.public.module import FooClass

# Wrong:
import _compat
from some._protected.module import BarClass
from some.module import _protected

New in version 0.3.0.

Changed in version 0.11.0.

error_template = 'Found protected module import'

code = 436

previous_codes = {440}

class ProtectedAttributeViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use protected attributes and methods.

Reasoning:

When using protected attributes and method we break a contract that authors of this class enforce. This way we are not respecting encapsulation and it may break our code at any moment.

Solution:

Do not use protected attributes and methods. Respect the encapsulation.

Example:

# Correct:
self._protected = 1
cls._hidden_method()
some.public()
super()._protected()

# Wrong:
print(some._protected)
instance._hidden()
self.container._internal = 10

Note, that it is possible to use protected attributes with self, cls, and super() as base names. We allow this so you can create and use protected attributes and methods inside the class context. This is how protected attributes should be used.

New in version 0.3.0.

Changed in version 0.11.0.

error_template = 'Found protected attribute usage: {0}'

code = 437

previous_codes = {441}

class StopIterationInsideGeneratorViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to raise StopIteration inside generators.

Reasoning:

StopIteration should not be raised explicitly in generators.

Solution:

Use return statement to get out of a generator.

Example:

# Correct:
def some_generator():
    if some_value:
        return
    yield 1

# Wrong:
def some_generator():
    if some_value:
        raise StopIteration
    yield 1

See also

https://docs.python.org/3/library/exceptions.html#StopIteration

New in version 0.12.0.

error_template = 'Found `StopIteration` raising inside generator'

code = 438

class WrongUnicodeEscapeViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.TokenizeViolation

Forbids to use unicode escape sequences in binary strings.

Reasoning:

Binary strings do not work with unicode. Having unicode escape characters in there means that you have an error in your code.

Solution:

Use regular strings when escaping unicode strings.

Example:

# Correct:
escaped = '\u0041'  # equals to 'A'

# Wrong:
escaped = b'\u0040'  # equals to b'\\u0040'

New in version 0.12.0.

error_template = 'Found unicode escape in a binary string: {0}'

code = 439

class BlockAndLocalOverlapViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to local and block variables to overlap.

What we call local variables:

1. Assigns and annotations

2. Function arguments (they are local to the function body)

What we call block variables:

1. Imports

2. Functions and async functions definitions

3. Classes, methods, and async methods definitions

4. For and async for loops variables

5. Except block exception aliases

We allow local variables to overlap theirselfs, we forbid block varibals to overlap theirselfs.

Reasoning:

A lot of complex errors might happen when you shadow local varibales with block variables or when you shadow block variables with local variables.

Solution:

Use names that do not overlap.

Example:

# Correct:
my_value = 1
my_value = my_value + 1

# Wrong:
import my_value
my_value = 1  # overlaps with import

See also

https://github.com/satwikkansal/wtfPython#-explanation-20

New in version 0.12.0.

error_template = 'Found block variables overlap: {0}'

code = 440

class ControlVarUsedAfterBlockViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use control variables after the block body.

What we call block control variables:

1. for loop unpacked variables

2. with context variables

3. except exception names

Reasoning:

Variables leaking from the blocks can damage your logic. It might not contain what you think they contain. Some variables even might be deleted right after the block, just like in except Exception as exc: where exc won’t be in scope after except body.

Solution:

Use names inside the scope they are defined. Create new functions to return values in case you need to use block variables: when searching for a value, etc.

Example:

# Correct:
for my_item in collection:
    print(my_item)

# Wrong:
for my_item in collection:
    ...
print(my_item)

See also

https://github.com/satwikkansal/wtfPython#-explanation-32

New in version 0.12.0.

error_template = 'Found control variable used after block: {0}'

code = 441

class OuterScopeShadowingViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to shadow variables from outer scopes.

We check function, method, and module scopes. While we do not check class scope. Because class level constants are not available via regular name, and they are scope to ClassName.var_name.

Reasoning:

Shadowing can lead you to a big pile of strage and unexpected bugs.

Solution:

Use different names and do not allow scoping.

Example:

# Correct:
def test(): ...

def other():
    test1 = 1

# Wrong:
def test(): ...

def other():
    test = 1  # shadows `test()` function

New in version 0.12.0.

error_template = 'Found outer scope names shadowing: {0}'

code = 442

class UnhashableTypeInHashViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use exlicit unhashable types as set items and dict keys.

Reasoning:

This will resolve in TypeError in runtime.

Solution:

Use hashable types to define set items and dict keys.

Example:

# Correct:
my_dict = {1: {}, (1, 2): [], (2, 3): {1, 2}}

# Wrong:
my_dict = {[1, 2]: [], {2, 3}: {1, 2}}

New in version 0.12.0.

error_template = 'Found unhashable item'

code = 443

class WrongKeywordConditionViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use exlicit falsly-evaluated conditions with several keywords.

We check:

• ast.While

• ast.Assert

We only check constants. We do not check variables, attributes, calls, etc.

Reasoning:

Some conditions clearly tell us that this node won’t work correctly. So, we need to check that we can fix that.

Solution:

Remove the unreachable node, or change the condition item.

Example:

# Correct:
assert some_variable

while True:
    ...

# Wrong:
assert []

while False:
    ...

New in version 0.12.0.

error_template = 'Found wrong keyword condition: {0}'

code = 444