OOP

These checks ensures that you use Python’s version of OOP correctly.

There are different gotchas in Python to write beatiful classes and using objects correctly. That’s the place we collect these kind of rules.

Summary

BuiltinSubclassViolation

Forbids to subclass lowercase builtins.

ShadowedClassAttributeViolation

Forbids to shadow class level attributes with instance level attributes.

StaticMethodViolation

Forbids to use @staticmethod decorator.

BadMagicMethodViolation

Forbids to use some magic methods.

WrongClassBodyContentViolation

Forbids to use incorrect nodes inside class definitions.

MethodWithoutArgumentsViolation

Forbids to have methods without any arguments.

WrongBaseClassViolation

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

WrongSlotsViolation

Forbids to have incorrect __slots__ definition.

WrongSuperCallViolation

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

DirectMagicAttributeAccessViolation

Forbids to use direct magic attributes and methods.

AsyncMagicMethodViolation

Forbids to make some magic methods async.

YieldMagicMethodViolation

Forbids to use yield inside of several magic methods.

UselessOverwrittenMethodViolation

Forbids to have useless overwritten methods.

WrongSuperCallAccessViolation

Forbids to use super() with incorrect methods or properties access.

Respect your objects

class BuiltinSubclassViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to subclass lowercase builtins.

We forbid to subclass builtins like int, str, bool, etc. We allow to subclass object and type, warnings, and exceptions.

See ALLOWED_BUILTIN_CLASSES for the whole list of whitelisted names.

Reasoning:

It is almost never a good idea (unless you do something sneaky) to subclass primitive builtins.

Solution:

Use custom objects around some wrapper. Use magic methods to emulate the desired behaviour.

Example:

# Correct:
class Some(object): ...
class MyValueException(ValueError): ...

# Wrong:
class MyInt(int): ...

New in version 0.10.0.

Changed in version 0.11.0.

error_template = 'Found subclassing a builtin: {0}'
code = 600
previous_codes = {426}
class ShadowedClassAttributeViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to shadow class level attributes with instance level attributes.

Reasoning:

This way you will have two attributes inside your __mro__ chain: one from instance and one from class. It might cause errors. Needless to say, that this is just pointless to do so.

Solution:

Use either class attributes or instance attributes. Use ClassVar type on fields that are declared as class attributes.

Note, that we cannot find shadowed attributes that are defined in parent classes. That’s where ClassVar is required for mypy to check it for you.

Example:

# Correct:
from typing import ClassVar

class First(object):
    field: ClassVar[int] = 1

class Second(object):
    field: int

    def __init__(self) -> None:
        self.field = 1

# Wrong:
class Some(object):
    field = 1

    def __init__(self) -> None:
        self.field = 1

New in version 0.10.0.

Changed in version 0.11.0.

error_template = 'Found shadowed class attribute: {0}'
code = 601
previous_codes = {427}
class StaticMethodViolation(node, text=None, baseline=None)[source]

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.

Changed in version 0.11.0.

error_template = 'Found using `@staticmethod`'
code = 602
previous_codes = {433}
class BadMagicMethodViolation(node, text=None, baseline=None)[source]

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.

Changed in version 0.11.0.

error_template = 'Found using restricted magic method: {0}'
code = 603
previous_codes = {434}
class WrongClassBodyContentViolation(node, text=None, baseline=None)[source]

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.

Changed in version 0.11.0.

error_template = 'Found incorrect node inside `class` body'
code = 604
previous_codes = {452}
class MethodWithoutArgumentsViolation(node, text=None, baseline=None)[source]

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.

Changed in version 0.11.0.

error_template = 'Found method without arguments: {0}'
code = 605
previous_codes = {453}
class WrongBaseClassViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

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

We only check base classes and not keywords. They can be anything you need.

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. Use annotation future import in case you use strings in 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.

Changed in version 0.11.0.

Changed in version 0.12.0.

error_template = 'Found incorrect base class'
code = 606
previous_codes = {454}
class WrongSlotsViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have incorrect __slots__ definition.

Things that this rule checks:

  • That __slots__ is a tuple, name, attribute, star, or call

  • That __slots__ do not have duplicates

  • That __slots__ do not have empty strings or invalid python names

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. Use snake_case to define attributes in __slots__.

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.

Changed in version 0.11.0.

Changed in version 0.12.0.

error_template = 'Found incorrect `__slots__` syntax'
code = 607
previous_codes = {455}
class WrongSuperCallViolation(node, text=None, baseline=None)[source]

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.

Changed in version 0.11.0.

error_template = 'Found incorrect `super()` call: {0}'
code = 608
previous_codes = {456}
class DirectMagicAttributeAccessViolation(node, text=None, baseline=None)[source]

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.

Changed in version 0.11.0.

error_template = 'Found direct magic attribute usage: {0}'
code = 609
previous_codes = {462}
class AsyncMagicMethodViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to make some magic methods async.

We allow to make __anext__, __aenter__, __aexit__ async. We also allow custom magic methods to be async.

See ASYNC_MAGIC_METHODS_BLACKLIST for the whole list of blacklisted async magic methods.

Reasoning:

Defining the magic methods as async which are not supposed to be async would not work as expected.

Solution:

Do not make this magic method async.

Example:

# Correct:
class Test(object):
    def __lt__(self, other): ...

# Wrong:
class Test(object):
    async def __lt__(self, other): ...

New in version 0.12.0.

error_template = 'Found forbidden `async` magic method usage: {0}'
code = 610
class YieldMagicMethodViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use yield inside of several magic methods.

We allow to make __iter__ a generator. See YIELD_MAGIC_METHODS_BLACKLIST for the whole list of blacklisted generator magic methods.

Reasoning:

Python’s datamodel is strict. You cannot make generators from random magic methods. This rule enforces it.

Solution:

Remove yield from a magic method or rename it to be a custom method.

Example:

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

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

New in version 0.3.0.

Changed in version 0.11.0.

Changed in version 0.12.0.

error_template = 'Found forbidden `yield` magic method usage'
code = 611
previous_codes = {435, 439}
class UselessOverwrittenMethodViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have useless overwritten methods.

Reasoning:

Overwriting method without any changes does not have any positive impact.

Solution:

Do not overwrite method in case you do not want to do any changes inside it.

Example:

# Correct:
class Test(Base):
    ...

# Wrong:
class Test(object):
    def method(self, argument):
        return super().method(argument)

New in version 0.12.0.

error_template = 'Found useless overwritten method: {0}'
code = 612
class WrongSuperCallAccessViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use super() with incorrect methods or properties access.

Reasoning:

Can only use super() method that matches the following context. super().some() and super().some in Child.some(), and super().prop and super().prop() in Child.prop

Solution:

Use super() methods and properties with the correct context.

Example:

# Correct:
class Child(Parent):
    def some_method(self):
        original = super().some_method()

# Wrong:
class Child(Parent):
    def some_method(self):
        other = super().other_method()

New in version 0.13.0.

error_template = 'Found incorrect `super()` call context: incorrect name access'
code = 613