Complexity

These checks find flaws in your application design.

We try to stick to “the magical 7 ± 2 number” when counting things. https://en.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus_or_Minus_Two

That’s how many objects we can keep in our memory at a time. We try hard not to exceed the memory capacity limit.

You can also find interesting reading about “Cognitive complexity”: https://www.sonarsource.com/docs/CognitiveComplexity.pdf

Note

Simple is better than complex. Complex is better than complicated.

Summary

JonesScoreViolation

Forbids to have modules with complex lines.

TooManyImportsViolation

Forbids to have modules with too many imports.

TooManyModuleMembersViolation

Forbids to have many classes and functions in a single module.

TooManyImportedNamesViolation

Forbids to have modules with too many imported names.

OverusedExpressionViolation

Forbids to have overused expressions in a module, function or method.

TooManyLocalsViolation

Forbids to have too many local variables in the unit of code.

TooManyArgumentsViolation

Forbids to have too many arguments for a function or method.

TooManyReturnsViolation

Forbids placing too many return statements into the function.

TooManyExpressionsViolation

Forbids putting too many expressions in a single function.

TooManyMethodsViolation

Forbids to have many methods in a single class.

TooManyBaseClassesViolation

Restrict the maximum number of base classes.

TooManyDecoratorsViolation

Restrict the maximum number of decorators.

TooManyAwaitsViolation

Forbids placing too many await expressions into a function.

TooManyAssertsViolation

Forbids placing too many asseert statements into a function.

TooDeepAccessViolation

Forbids to have consecutive expressions with too deep access level.

TooDeepNestingViolation

Forbids nesting blocks too deep.

LineComplexityViolation

Forbids to have complex lines.

TooManyConditionsViolation

Forbids to have conditions with too many logical operators.

TooManyElifsViolation

Forbids to use many elif branches.

TooManyForsInComprehensionViolation

Forbids to have too many for statement within a comprehension.

TooManyExceptCasesViolation

Forbids to have too many except cases in a single try clause.

OverusedStringViolation

Forbids to over-use string constants.

TooLongYieldTupleViolation

Forbids to yield too long tuples.

TooLongCompareViolation

Forbids to have too long compare expressions.

TooLongTryBodyViolation

Forbids to have try blocks with too long bodies.

TooManyPublicAttributesViolation

Forbids to have instances with too many public attributes.

CognitiveComplexityViolation

Forbids to have functions with too high cognitive complexity.

CognitiveModuleComplexityViolation

Forbids to have modules with too high average cognitive complexity.

TooLongCallChainViolation

Forbids too long call chains.

TooComplexAnnotationViolation

Forbids too complex annotations.

TooManyImportedModuleMembersViolation

Forbids from ...import ... with too many imported names..

TooLongTupleUnpackViolation

Forbids using too many variables to unpack a tuple.

Module complexity

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

Bases: wemake_python_styleguide.violations.base.SimpleViolation

Forbids to have modules with complex lines.

We are using Jones Complexity algorithm to count module’s score. See LineComplexityViolation for details of per-line-complexity. How it is done: we count complexity per line, then measuring the median complexity across the lines in the whole module.

Reasoning:

Having complex modules will decrease your code maintainability.

Solution:

Refactor the module contents.

Configuration:

This rule is configurable with --max-jones-score. Default: 12

New in version 0.1.0.

error_template = 'Found module with high Jones Complexity score: {0}'
code = 200
class TooManyImportsViolation(node=None, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.SimpleViolation

Forbids to have modules with too many imports.

Namespaces are one honking great idea – let’s do more of those!

Reasoning:

Having too many imports without prefixes is quite expensive. You have to memorize all the source locations of the imports. And sometimes it is hard to remember what kind of functions and classes are already injected into your context.

It is also a questionable design if a single module has a lot of imports. Why a single module has so many dependencies? So, it becomes too coupled.

Solution:

Refactor the imports to import a common namespace. Something like from package import module and then use it like module.function().

Or refactor your code and split the complex module into several ones.

We do not make any differences between import and from ... import ....

Configuration:

This rule is configurable with --max-imports. Default: 12

New in version 0.1.0.

error_template = 'Found module with too many imports: {0}'
code = 201
class TooManyModuleMembersViolation(node=None, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.SimpleViolation

Forbids to have many classes and functions in a single module.

Reasoning:

Having many classes and functions in a single module is a bad thing. Soon it will be hard to read through this code and understand it.

Solution:

It is better to split this module into several modules or a package.

We do not make any differences between classes and functions in this check. They are treated as the same unit of logic. We also do not care about functions and classes being public or not. However, methods are counted separately on a per-class basis.

Configuration:

This rule is configurable with --max-module-members. Default: 7

New in version 0.1.0.

error_template = 'Found too many module members: {0}'
code = 202
class TooManyImportedNamesViolation(node=None, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.SimpleViolation

Forbids to have modules with too many imported names.

Namespaces are one honking great idea – let’s do more of those!

Reasoning:

Having too many imported names without prefixes is quite expensive. You have to memorize all the source locations of the imports. And sometimes it is hard to remember what kind of functions and classes are already injected into your context.

It is also a questionable design if a single module has a lot of imports. Why a single module has so many dependencies? So, it becomes too coupled.

Solution:

Refactor the imports to import a common namespace. Something like from package import module and then use it like module.function().

Or refactor your code and split the complex module into several ones.

Example:

# Correct:
import module  # 1 imported name

# Wrong:
from module import func1, func2, ..., funcN  # N imported names

We do not make any differences between import and from ... import ....

Configuration:

This rule is configurable with --max-imported-names. Default: 50

New in version 0.12.0.

error_template = 'Found module with too many imported names: {0}'
code = 203
class OverusedExpressionViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have overused expressions in a module, function or method.

What do we call an “overused expression”? When you use any expression (like user_dict['age'] for example) inside your code, you always have to track that you are not using it “too much”. Because if that expression is everywhere inside your code, it is a sign of a problem. It means that you are missing an abstraction.

We check overused expression on two levels:

  • per each function

  • per all module

Related to TooManyExpressionsViolation.

Reasoning:

Overusing expression lead to losing the parts that can and should be refactored into variables, methods, and properties of objects.

Solution:

Refactor expressions to be an attribute, a method, or a new variable.

Configuration:

This rule is configurable with --max-module-expressions. Default: 7

And with --max-function-expressions. Default: 4

New in version 0.12.0.

Changed in version 0.14.0.

error_template = 'Found overused expression: {0}'
code = 204

Structure complexity

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

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have too many local variables in the unit of code.

Reasoning:

Having too many variables in a single function is a bad thing. Soon, you will find troubles to understand what this variable means. It will also become hard to name new variables.

Solution:

If you have too many variables in a function, you have to refactor it.

What counts as a local variable? We only count variable as local in the following case: it is assigned inside the function body. We do not count variables defined inside comprehensions as local variables, since it is impossible to use them outside of the comprehension.

Example:

def first_function(param):
    first_var = 1

def second_function(argument):
    second_var = 1
    argument = int(argument)
    third_var, _ = some_call()

In this example we will count as locals only several variables:

  1. first_var, because it is assigned inside the function’s body

  2. second_var, because it is assigned inside the function’s body

  3. argument, because it is reassigned inside the function’s body

  4. third_var, because it is assigned inside the function’s body

Please, note that _ is a special case. It is not counted as a local variable. Since by design it means: do not count me as a real variable.

Configuration:

This rule is configurable with --max-local-variables. Default: 5

New in version 0.1.0.

error_template = 'Found too many local variables: {0}'
code = 210
class TooManyArgumentsViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have too many arguments for a function or method.

Reasoning:

This is an indicator of a bad design. When a function requires many arguments it shows that it is required to refactor this piece of code. It also indicates that function does too many things at once.

Solution:

Split function into several functions. Then it will be easier to use them.

Configuration:

This rule is configurable with --max-arguments. Default: 5

New in version 0.1.0.

error_template = 'Found too many arguments: {0}'
code = 211
class TooManyReturnsViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids placing too many return statements into the function.

Reasoning:

When there are too many return keywords, functions are hard to test. They are also hard to read and hard to change and keep everything inside your head at once.

Solution:

Change your design. Split functions into multiple ones.

Configuration:

This rule is configurable with --max-returns. Default: 5

New in version 0.1.0.

error_template = 'Found too many return statements: {0}'
code = 212
class TooManyExpressionsViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids putting too many expressions in a single function.

This rule is quite similar to “max lines” in a function, but is much nicer. Because we don’t count lines, we count real code entities. This way adding just several extra empty lines for readability will never trigger this violation.

Related to OverusedExpressionViolation.

Reasoning:

When there are too many expressions it means that this specific function does too many things at once. It has too much logic.

Solution:

Split function into several functions, refactor your API.

Configuration:

This rule is configurable with --max-expressions. Default: 9

New in version 0.1.0.

error_template = 'Found too many expressions: {0}'
code = 213
class TooManyMethodsViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have many methods in a single class.

Reasoning:

Having too many methods might lead to the “God object”. This kind of objects can handle everything. So, in the end, your code becomes too hard to maintain and test.

Solution:

What to do if you have too many methods in a single class? Split this class into several classes. Then use composition or inheritance to refactor your code. This will protect you from “God object” anti-pattern.

We do not make any difference between instance and class methods. We also do not care about functions and classes being public or not. We also do not count inherited methods from parents. This rule does not count the attributes of a class.

Configuration:

This rule is configurable with --max-methods. Default: 7

New in version 0.1.0.

error_template = 'Found too many methods: {0}'
code = 214
class TooManyBaseClassesViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Restrict the maximum number of base classes.

Reasoning:

It is almost never possible to navigate to the desired method of a parent class when you need it with multiple mixins. It is hard to understand mro and super calls. Do not overuse this technique.

Solution:

Reduce the number of base classes. Use composition over inheritance.

Example:

# Correct:
class SomeClassName(First, Second, Mixin): ...

# Wrong:
class SomeClassName(
    FirstParentClass,
    SecondParentClass,
    ThirdParentClass,
    CustomClass,
    AddedClass,
 ): ...
Configuration:

This rule is configurable with --max-base-classes. Default: 3

New in version 0.3.0.

Changed in version 0.5.0.

error_template = 'Too many base classes: {0}'
code = 215
class TooManyDecoratorsViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Restrict the maximum number of decorators.

Reasoning:

When you are using too many decorators it means that you try to overuse the magic. You have to ask yourself: do I really know what happens inside this decorator tree? Typically, the answer will be “no”.

Solution:

Using too many decorators typically means that you try to configure the behavior from outside of the class. Do not do that too much. Split functions or classes into multiple ones. Use higher order decorators.

Configuration:

This rule is configurable with --max-decorators. Default: 5

This rule checks: functions, methods, and classes.

New in version 0.5.0.

error_template = 'Too many decorators: {0}'
code = 216
class TooManyAwaitsViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids placing too many await expressions into a function.

Reasoning:

When there are too many await keywords, functions are starting to get really complex. It is hard to tell where are we and what is going on.

Solution:

Change your design. Split functions into multiple ones.

Configuration:

This rule is configurable with --max-awaits. Default: 5

New in version 0.10.0.

error_template = 'Found too many await expressions: {0}'
code = 217
class TooManyAssertsViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids placing too many asseert statements into a function.

Reasoning:

When there are too many assert keywords, functions are starting to get really complex. It might indicate that your tests or contracts are too big.

Solution:

Create rich assert statements, use higher-level contracts, or create special guard functions.

Configuration:

This rule is configurable with --max-asserts. Default: 5

New in version 0.12.0.

error_template = 'Found too many `assert` statements: {0}'
code = 218
class TooDeepAccessViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have consecutive expressions with too deep access level.

We consider only these expressions as accesses:

  • ast.Subscript

  • ast.Attribute

We do not treat ast.Call as an access, since there are a lot of call-based APIs like Django ORM, builder patterns, etc.

Reasoning:

Having too deep access level indicates a bad design and overcomplicated data without proper API.

Solution:

Split the expression into variables, functions or classes. Refactor the API for your data layout.

Example:

# Correct: access level = 4
self.attr.inner.wrapper[1]

# Correct: access level = 1
manager.filter().exclude().annotate().values().first()

# Wrong: access level = 5
self.attr.inner.wrapper.method.call()

# Wrong: access level = 5
# `obj` has access level of 2:
# `.attr`, `.call`
# `call()` has access level of 5:
# `.other`, `[0]`, `.field`, `.type`, `.boom`
obj.attr.call().other[0].field.type.boom
Configuration:

This rule is configurable with --max-access-level. Default: 4

New in version 0.12.0.

error_template = 'Found too deep access level: {0}'
code = 219
class TooDeepNestingViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids nesting blocks too deep.

Reasoning:

If nesting is too deep that indicates usage of complex logic and language constructions. This means that our design is not suited to handle such construction.

Solution:

We need to refactor our complex construction into simpler ones. We can use new functions or different constructions.

New in version 0.1.0.

Changed in version 0.5.0.

error_template = 'Found too deep nesting: {0}'
code = 220
class LineComplexityViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have complex lines.

We are using Jones Complexity algorithm to count complexity. What is Jones Complexity? It is a simple yet powerful method to count the number of ast nodes per line. If the complexity of a single line is higher than a threshold, then an error is raised.

What nodes do we count? All except the following:

  1. modules

  2. function and classes, since they are checked differently

  3. type annotations, since they do not increase the complexity

Reasoning:

Having a complex line indicates that you somehow managed to put too much logic inside a single line. At some point in time, you will no longer be able to understand what this line means and what it does.

Solution:

Split a single line into several lines: by creating new variables, statements or functions. Note, this might trigger new complexity issues. With this technique, a single new node in a line might trigger a complex refactoring process including several modules.

Configuration:

This rule is configurable with --max-line-complexity. Default: 14

New in version 0.1.0.

error_template = 'Found line with high Jones Complexity: {0}'
code = 221
class TooManyConditionsViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have conditions with too many logical operators.

We use 4 as a default value.

Reasoning:

When reading through the complex conditions you will fail to understand all the possible branches. And you will end up putting debug breakpoint on this line just to figure out how it works.

Solution:

We can reduce the complexity of a single if by doing two things: creating new variables or creating nested if statements. Both of these actions will trigger other complexity checks.

We count and and or keywords as conditions.

New in version 0.1.0.

Changed in version 0.5.0.

error_template = 'Found a condition with too much logic: {0}'
code = 222
class TooManyElifsViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use many elif branches.

We use 3 as a default value.

Reasoning:

This rule is specifically important because of many elif branches indicate a complex flow in your design: you are reimplementing switch in python.

Solution:

There are different design patterns to use instead. For example, you can use some interface that just call a specific method without if. Or separate your if into multiple functions.

New in version 0.1.0.

Changed in version 0.5.0.

error_template = 'Found too many `elif` branches: {0}'
code = 223
class TooManyForsInComprehensionViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have too many for statement within a comprehension.

Reasoning:

When reading through the complex comprehension you will fail to understand it.

Solution:

We can reduce the complexity of comprehension by reducing the amount of for statements. Refactor your code to use several for loops, comprehensions, or different functions.

Example:

# Wrong:
ast_nodes = [
    target
    for assignment in top_level_assigns
    for target in assignment.targets
    for _ in range(10)
]

New in version 0.3.0.

error_template = 'Found a comprehension with too many `for` statements'
code = 224
class TooManyExceptCasesViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have too many except cases in a single try clause.

We use 3 as a default value.

Reasoning:

Handling too many exceptions in a single place is a good indicator of a bad design. Since this way, one controlling structure will become too complex. And you will need to test a lot of paths your application might go.

Solution:

We can reduce the complexity of this case by splitting it into multiple try cases, functions or using a decorator to handle different exceptions.

New in version 0.7.0.

error_template = 'Found too many `except` cases: {0}'
code = 225
class OverusedStringViolation(node=None, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.MaybeASTViolation

Forbids to over-use string constants.

We allow to use strings without any restrictions as annotations for variables, arguments, return values, and class attributes.

Reasoning:

When some string is used more than several time in your code, it probably means that this string is a meaningful constant. And should be treated like one.

Solution:

Deduplicate you string usages by defining new functions or constants.

Configuration:

This rule is configurable with --max-string-usages. Default: 3

New in version 0.10.0.

error_template = 'Found string constant over-use: {0}'
code = 226
class TooLongYieldTupleViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to yield too long tuples.

Reasoning:

Long yield tuples complicate generator using. This rule helps to reduce complication.

Solution:

Use lists of similar type or wrapper objects.

New in version 0.10.0.

error_template = 'Found too long yield tuple: {0}'
code = 227
class TooLongCompareViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have too long compare expressions.

Reasoning:

To long compare expressions indicate that there’s something wrong going on in the code. Compares should not be longer than 3 or 4 items.

Solution:

Use several conditions, seprate variables, or functions.

New in version 0.10.0.

error_template = 'Found too long compare'
code = 228
class TooLongTryBodyViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have try blocks with too long bodies.

Reasoning:

Having too many statements inside your try block can lead to situations when some different statement raises an exception and you are not aware of it since it is not expected.

Solution:

Move things out of the try block or create new functions. The less lines you have in your try block - the safer you are from accidental errors.

Configuration:

This rule is configurable with --max-try-body-length. Default: 1

New in version 0.12.0.

error_template = 'Found too long ``try`` body length: {0}'
code = 229
class TooManyPublicAttributesViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have instances with too many public attributes.

We only check static definitions in a form of self.public = .... We do not count parent attributes. We do not count properties. We do not count annotations. We do not count class attributes. We do not count duplicates.

Reasoning:

Having too many public instance attributes means that your class is too complex in terms of coupling. Other classes and functions will rely on these concrete fields instead of better abstraction layers.

Solution:

Make some attributes protected. Split this class into several ones. If class is a Data Transfer Object, then use @dataclass decorator.

Configuration:

This rule is configurable with --max-attributes. Default: 6

New in version 0.12.0.

error_template = 'Found too many public instance attributes: {0}'
code = 230
class CognitiveComplexityViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have functions with too high cognitive complexity.

Reasoning:

People are not great at reading and iterpretating code in their heads. That’s why code with a lot of nested loops, conditions, exceptions handlers, and context managers is hard to read and understand.

Solution:

Rewrite your code to be simplier. Use flat structures and conditions, remove nested loops.

Configuration:

This rule is configurable with --max-cognitive-score. Default: 12

New in version 0.13.0.

error_template = 'Found too high function cognitive complexity: {0}'
code = 231
class CognitiveModuleComplexityViolation(node=None, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.SimpleViolation

Forbids to have modules with too high average cognitive complexity.

Reasoning:

Modules with lots of functions might hide cognitive complexity inside many small and relatevely simple functions.

Solution:

Rewrite your code to be simplier. Or use several modules.

Configuration:

This rule is configurable with --max-cognitive-average. Default: 8

New in version 0.13.0.

error_template = 'Found too high module cognitive complexity: {0}'
code = 232
class TooLongCallChainViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids too long call chains.

Reasoning:

Too long call chains are overcomplicated and indicators of bad API design.

Solution:

Split the expression into variables, functions or classes. Refactor the API to allow higher-level access to functions.

Configuration:

This rule is configurable with --max-call-level. Default: 3

New in version 0.13.0.

error_template = 'Found too long call chain length: {0}'
code = 233
class TooComplexAnnotationViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids too complex annotations.

Annotation complexity is maximum annotation nesting level. Example: List[int] has complexity of 2 and Tuple[List[Optional[str]], int] has complexity of 4.

Reasoning:

Too complex annotations make your types unreadable. And make developers afraid of types.

Solution:

Create type aliases. And use them a lot!

Configuration:

This rule is configurable with --max-annotation-complexity. Default: 3

New in version 0.14.0.

error_template = 'Found too complex annotation: {0}'
code = 234
class TooManyImportedModuleMembersViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids from ... import ... with too many imported names.

Reasoning:

Importing too many names from one import is easy way to cause violation WPS203 - too many imported names.

Solution:

Refactor the imports to import a common namespace. Something like from package import module and then use it like module.function().

Example:

# Correct:
import module  # 1 imported name

# Wrong:
from module import func1, func2, ..., funcN  # N imported names
Configuration:

This rule is configurable with --max-import-from-members. Default: 8

New in version 0.14.0.

error_template = 'Found too many imported names from a module: {0}'
code = 235
class TooLongTupleUnpackViolation(node, text=None, baseline=None)[source]

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids using too many variables to unpack a tuple.

Reasoning:

The order and meaning are hard to remember.

Solution:

If you have more than 2 values in a tuple, consider using typing.NamedTuple or a dataclass instead.

Example:

# Correct:
result = foo()

# Wrong:
a, b, c, d, e = foo()
Configuration:

This rule is configurable with --max-tuple-unpack-length. Default: 4

New in version 0.15.0.

error_template = 'Found too many variables used to unpack a tuple: {0}'
code = 236