Best practices¶

These checks ensures that you follow the best practices.

Note

Explicit is better than implicit. Simple is better than complex. Complex is better than complicated. Readability counts. Special cases aren’t special enough to break the rules. In the face of ambiguity, refuse the temptation to guess. There should be one– and preferably only one –obvious way to do it.

Summary¶

`WrongMagicCommentViolation`	Restricts to use several control (or magic) comments.
`WrongDocCommentViolation`	Forbids to use empty doc comments (`#:`).
`WrongModuleMetadataViolation`	Forbids to have some module level variables.
`EmptyModuleViolation`	Forbids to have empty modules.
`InitModuleHasLogicViolation`	Forbids to have logic inside `__init__` module.
`WrongKeywordViolation`	Forbids to use some keywords from `python`.
`WrongFunctionCallViolation`	Forbids to call some built-in functions.
`FutureImportViolation`	Forbids to use `__future__` imports.
`RaiseNotImplementedViolation`	Forbids to use `NotImplemented` error.
`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.

Comments¶

class WrongMagicCommentViolation(node=None, text=None)[source]¶

Bases: wemake_python_styleguide.violations.base.SimpleViolation

Restricts to use several control (or magic) comments.

We do not allow to use:

# noqa comment without specified violations
# 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

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

Note

Returns Z400 as error code

error_template = 'Found wrong magic comment: {0}'¶: Error message shown to the user.

class WrongDocCommentViolation(node, text=None)[source]¶

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']

Note

Returns Z401 as error code

error_template = 'Found wrong doc comment'¶: Error message shown to the user.

Modules¶

class WrongModuleMetadataViolation(node, text=None)[source]¶

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 BAD_MODULE_METADATA_VARIABLES for full list of bad names.

Example:

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

Note

Returns Z410 as error code

error_template = 'Found wrong metadata variable {0}'¶: Error message shown to the user.

class EmptyModuleViolation(node, text=None)[source]¶

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have empty modules.

Reasoning:

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

Solution:

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

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

Note

Returns Z411 as error code

error_template = 'Found empty module'¶: Error message shown to the user.

class InitModuleHasLogicViolation(node, text=None)[source]¶

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have logic inside __init__ module.

Reasoning:

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

you are keeping some outdated stuff there, you need to refactor
you are placing this logic into the wrong file, just create another one
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:

comments, since they are dropped before AST comes in play
docs string, because sometimes it is required to state something

Note

Returns Z412 as error code

error_template = 'Found `__init__` module with logic'¶: Error message shown to the user.

Builtins¶

class WrongKeywordViolation(node, text=None)[source]¶

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use some keywords from python.

Reasoning:: We believe that some keywords are anti-patterns. They promote bad-practices like global and pass, or just not user-friendly like del.
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.

Example:

# Wrong:
pass
del
nonlocal
global

Note

Returns Z420 as error code

error_template = 'Found wrong keyword "{0}"'¶: Error message shown to the user.

class WrongFunctionCallViolation(node, text=None)[source]¶

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 BAD_FUNCTIONS for the full list of blacklisted functions.

Note

Returns Z421 as error code

error_template = 'Found wrong function call "{0}"'¶: Error message shown to the user.

class FutureImportViolation(node, text=None)[source]¶

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

Note

Returns Z422 as error code

error_template = 'Found future import "{0}"'¶: Error message shown to the user.

class RaiseNotImplementedViolation(node, text=None)[source]¶

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

Note

Returns Z423 as error code

error_template = 'Found raise NotImplemented'¶: Error message shown to the user.

Design¶

class NestedFunctionViolation(node, text=None)[source]¶

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():
        ...

Note

Returns Z430 as error code

error_template = 'Found nested function "{0}"'¶: Error message shown to the user.

class NestedClassViolation(node, text=None)[source]¶

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):
        ...

Note

Returns Z431 as error code

error_template = 'Found nested class "{0}"'¶: Error message shown to the user.

class MagicNumberViolation(node, text=None)[source]¶

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

Note

Returns Z432 as error code

error_template = 'Found magic number: {0}'¶: Error message shown to the user.

class StaticMethodViolation(node, text=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.

Note

Returns Z433 as error code

error_template = 'Found using `@staticmethod`'¶: Error message shown to the user.

class BadMagicMethodViolation(node, text=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 you code to use custom methods instead. It will give more context to your app.

See BAD_MAGIC_METHODS for the full blacklist of the magic methods.

Note

Returns Z434 as error code

error_template = 'Found using restricted magic method "{0}"'¶: Error message shown to the user.

class NestedImportViolation(node, text=None)[source]¶

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 you 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

Note

Returns Z435 as error code

error_template = 'Found nested import "{0}"'¶: Error message shown to the user.

Best practices¶

Summary¶

Comments¶

Modules¶

Builtins¶

Design¶

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