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.
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.
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.
NestedFunctionViolation	Forbids to have nested functions.
NestedClassViolation	Forbids to use nested classes.
MagicNumberViolation	Forbids to use magic numbers in your code.
StaticMethodViolation	Forbids to use @staticmethod decorator.
BadMagicMethodViolation	Forbids to use some magic methods.
NestedImportViolation	Forbids to have nested imports in functions.
RedundantLoopElseViolation	Forbids to use else without break in a loop.
RedundantFinallyViolation	Forbids to use finally in try block without except block.
ReassigningVariableToItselfViolation	Forbids to assign variable to itself.
YieldInsideInitViolation	Forbids to use yield inside of __init__ method.
ProtectedModuleViolation	Forbids to import protected modules.
ProtectedAttributeViolation	Forbids to use protected attributes and methods.
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.
IncorrectUnpackingViolation	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.
NonUniqueItemsInSetViolation	Forbids to have duplicate items in set literals.
BaseExceptionSubclassViolation	Forbids to have duplicate items in set literals.
SimplifiableIfViolation	Forbids to have simplifiable if conditions.
IncorrectClassBodyContentViolation	Forbids to use incorrect nodes inside class definitions.
MethodWithoutArgumentsViolation	Forbids to have methods without any arguments.
IncorrectBaseClassViolation	Forbids to have anything else than a class as a base class.
IncorrectSlotsViolation	Forbids to have incorrect __slots__ definition.
IncorrectSuperCallViolation	Forbids to use super() with parameters or outside of methods.
RedundantReturningElseViolation	Forbids to use redundant else cases in returning functions.
TryExceptMultipleReturnPathViolation	Forbids to use multiple return path with try / except case.
ComplexDefaultValuesViolation	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.
DirectMagicAttributeAccessViolation	Forbids to use direct magic attributes and methods.
NegatedConditionsViolation	Forbids to use negated conditions together with else clause.
NestedTryViolation	Forbids to use nested try blocks.
MultilineConditionsViolation	Forbid multiline conditions.

Comments

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

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 default value.

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

Modules

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

Builtins

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

Design

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 StaticMethodViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use @staticmethod decorator.

Reasoning:

Static methods are not required to be inside the class. Because they even do not have access to the current instance.

Solution:

Use instance methods, @classmethod, or functions instead.

New in version 0.1.0.

error_template = 'Found using `@staticmethod`'

code = 433

class BadMagicMethodViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use some magic methods.

Reasoning:

We forbid to use magic methods related to the forbidden language parts. Likewise, we forbid to use del keyword, so we forbid to use all magic methods related to it.

Solution:

Refactor your code to use custom methods instead. It will give more context to your app.

See MAGIC_METHODS_BLACKLIST for the full blacklist of the magic methods.

New in version 0.1.0.

See also

https://www.youtube.com/watch?v=F6u5rhUQ6dU

error_template = 'Found using restricted magic method: {0}'

code = 434

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.

See also

https://github.com/seddonym/layer_linter

error_template = 'Found nested import'

code = 435

class RedundantLoopElseViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use else without break in a loop.

We use the same logic for for and while loops.

Reasoning:

When there’s no break keyword in loop’s body it means that else will always be called. This rule will reduce complexity, improve readability, and protect from possible errors.

Solution:

Refactor your else case logic to be inside the loop’s body. Or right after it.

Example:

# Correct:
for letter in 'abc':
    if letter == 'b':
        break
else:
    print('"b" is not found')

for letter in 'abc':
    print(letter)
print('always called')

# Wrong:
for letter in 'abc':
    print(letter)
else:
    print('always called')

New in version 0.3.0.

error_template = 'Found `else` in a loop without `break`'

code = 436

class RedundantFinallyViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use finally in try block without except block.

Reasoning:

This rule will reduce complexity and improve readability.

Solution:

Refactor your try logic. Replace the try-finally statement with a with statement.

Example:

# Correct:
with open("filename") as f:
    f.write(...)

# Wrong:
try:
    f = open("filename")
    f.write(...)
finally:
    f.close()

New in version 0.3.0.

error_template = 'Found `finally` in `try` block without `except`'

code = 437

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.

error_template = 'Found reassigning variable to itself'

code = 438

class YieldInsideInitViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use yield inside of __init__ method.

Reasoning:

__init__ should be used to initialize new objects. It shouldn’t yield anything because it should return None by the convention.

Example:

 # Correct:
class Example(object):
    def __init__(self):
        self._public_items_count = 0

# Wrong:
class Example(object):
    def __init__(self):
        yield 10

New in version 0.3.0.

error_template = 'Found `yield` inside `__init__` method'

code = 439

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.

error_template = 'Found protected module import'

code = 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.

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

code = 441

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.

See also

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

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

code = 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.

error_template = 'Found unreachable code'

code = 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.

error_template = 'Found statement that has no effect'

code = 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.

error_template = 'Found multiple assign targets'

code = 445

class IncorrectUnpackingViolation(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.

error_template = 'Found incorrect unpacking target'

code = 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.

error_template = 'Found duplicate exception: {0}'

code = 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')
['a', None, 'b', None]

>>> list([(yield letter) for letter in 'ab'])
['a', 'b']

See also

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

New in version 0.7.0.

error_template = 'Found `yield` inside comprehension'

code = 448

class NonUniqueItemsInSetViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have duplicate items in set literals.

Reasoning:

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

Solution:

Remove the 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

• containers: lists, dicts, tuples, sets

New in version 0.7.0.

error_template = 'Found non-unique item in `set` literal: {0}'

code = 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.

error_template = 'Found exception inherited from `BaseException`'

code = 450

class SimplifiableIfViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have simplifiable if conditions.

Reasoning:

This complex construction can cause frustration among other developers. It is longer, more verbose, and more complex.

Solution:

Use bool() to convert test values to boolean values. Or just leave it as it is in case when your test already returns a boolean value. Use can also use not keyword to switch boolean values.

Example:

# Correct:
my_bool = bool(some_call())
other_value = 8 if some_call() else None

# Wrong:
my_bool = True if some_call() else False

We only check if nodes where True and False values are used. We check both if nodes and if expressions.

New in version 0.7.0.

error_template = 'Found simplifiable `if` condition'

code = 451

class IncorrectClassBodyContentViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use incorrect nodes inside class definitions.

Reasoning:

Python allows us to have conditions, context managers, and even infinite loops inside class definitions. On the other hand, only methods, attributes, and docstrings make sense. So, we discourage using anything except these nodes in class bodies.

Solution:

If you have complex logic inside your class definition, most likely that you do something wrong. There are different options to refactor this mess. You can try metaclasses, decorators, builders, and other patterns.

Example:

# Wrong:
class Test(object):
    for _ in range(10):
        print('What?!')

We also allow some nested classes, check out NestedClassViolation for more information.

New in version 0.7.0.

error_template = 'Found incorrect node inside `class` body'

code = 452

class MethodWithoutArgumentsViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have methods without any arguments.

Reasoning:

Methods without arguments are allowed to be defined, but almost impossible to use. Furthermore, they don’t have an access to self, so can not access the inner state of the object. It might be an intentional design or just a typo.

Solution:

Move any methods with arguments to raw functions. Or just add an argument if it is actually required.

Example:

# Correct:
class Test(object):
    def method(self): ...

# Wrong:
class Test(object):
    def method(): ...

New in version 0.7.0.

error_template = 'Found method without arguments: {0}'

code = 453

class IncorrectBaseClassViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have anything else than a class as a base class.

Reasoning:

In Python you can specify anything in the base classes slot. In runtime this expression will be evaluated and executed. We need to prevent dirty hacks in this field.

Solution:

Use only attributes, names, and types to be your base classes.

Example:

# Correct:
class Test(module.ObjectName, MixinName, keyword=True): ...
class GenericClass(Generic[ValueType]): ...

# Wrong:
class Test((lambda: object)()): ...

New in version 0.7.0.

Changed in version 0.7.1.

error_template = 'Found incorrect base class'

code = 454

class IncorrectSlotsViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have incorrect __slots__ definition.

Reasoning:

__slots__ is a very special attribute. It completely changes your class. So, we need to be careful with it. We should not allow anything rather than tuples to define slots, we also need to check that fields defined in __slots__ are unique.

Solution:

Use tuples with unique elements to define __slots__ attribute.

Example:

# Correct:
class Test(object):
    __slots__ = ('field1', 'field2')

class Other(Test):
    __slots__ = Test.__slots__ + ('child',)

# Wrong:
class Test(object):
    __slots__ = ['field1', 'field2', 'field2']

Note, that we do ignore all complex expressions for this field. So, we only check raw literals.

New in version 0.7.0.

error_template = 'Found incorrect `__slots__` syntax'

code = 455

class IncorrectSuperCallViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use super() with parameters or outside of methods.

Reasoning:

super() is a very special function. It implicitly relies on the context where it is used and parameters passed to it. So, we should be very careful with parameters and context.

Solution:

Use super() without arguments and only inside methods.

Example:

# Correct:
super().__init__()

# Wrong:
super(ClassName, self).__init__()

New in version 0.7.0.

error_template = 'Found incorrect `super()` call: {0}'

code = 456

class RedundantReturningElseViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use redundant else cases in returning functions.

We check single if statements that all contain return or raise or break statements with this rule. We do not check if statements with elif cases.

Reasoning:

Using extra else creates a situation when the whole node could and should be dropped without any changes in logic. So, we prefer to have less code than more code.

Solution:

Remove redundant else case.

Example:

# Correct:
def some_function():
    if some_call():
        return 'yeap'
    return 'nope'

# Wrong:
def some_function():
    if some_call():
        raise ValueError('yeap')
    else:
        raise ValueError('nope')

New in version 0.7.0.

error_template = 'Found redundant returning `else` statement'

code = 457

class TryExceptMultipleReturnPathViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use multiple return path with try / except case.

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:

# 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.

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

code = 458

class ComplexDefaultValuesViolation(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.

error_template = 'Found complex default value'

code = 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.

Example:

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

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

New in version 0.8.0.

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

code = 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.

Example:

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

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

New in version 0.8.0.

error_template = 'Found wrong context manager variable definition'

code = 461

class DirectMagicAttributeAccessViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use direct magic attributes and methods.

Reasoning:

When using direct magic attributes or method it means that you are doing something wrong. Magic methods are not suited to be directly called or accessed.

Solution:

Use special syntax constructs that will call underlying magic methods.

Example:

# Correct:
super().__init__()

# Wrong:
2..__truediv__(2)
d.__delitem__('a')

Note, that it is possible to use direct magic attributes with self, cls, and super() as base names. We allow this because a lot of internal logic relies on these methods.

New in version 0.8.0.

error_template = 'Found direct magic attribute usage: {0}'

code = 462

class NegatedConditionsViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use negated conditions together with else clause.

Reasoning:

It easier to read and name regular conditions. Not negated ones.

Solution:

Move actions from the negated if condition to the else condition.

Example:

# Correct:
if some == 1:
     ...
else:
     ...

if not some:
     ...

# Wrong:
if not some:
     ...
else:
     ...

New in version 0.8.0.

error_template = 'Found negated condition'

code = 463

class NestedTryViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use nested try blocks.

Notice, we check all possible slots for try block: 1. the try block itself 2. all except cases 3. else case 4. and finally case

Reasoning:

Nesting try blocks indicates that something really bad happens to your logic. Why does it require two separate exception handlers? It is a perfect case to refactor your code.

Solution:

Collapse two exception handlers together. Or create a separate function that will handle this second nested case.

Example:

# Wrong:
try:
    try:
        ...
    except SomeException:
        ...
except SomeOtherException:
    ...

try:
    ...
except SomeOtherException:
    try:
        ...
    except SomeException:
        ...

New in version 0.8.0.

error_template = 'Found nested `try` block'

code = 464

class MultilineConditionsViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbid multiline conditions.

Reasoning:

This way of writing conditions hides the inner complexity this line has. And it decreases readability of the code.

Solution:

Divide multiline conditions to some if condition. Or use variables.

Example:

# Correct:
if isinstance(node.test, ast.UnaryOp):
    if isinstance(node.test.op, ast.Not):
        ...


# Wrong:
if isinstance(node.test, ast.UnaryOp) and isinstance(
    node.test.op,
    ast.Not,
):
...

New in version 0.9.0.

error_template = 'Found multiline conditions'

code = 465