Complexity

These checks finds 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.
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 unit of code.
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.
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.

Module complexity

class JonesScoreViolation(node=None, text=None)

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.

See also

https://github.com/Miserlou/JonesComplexity

error_template = 'Found module with high Jones Complexity score: {0}'

code = 200

class TooManyImportsViolation(node=None, text=None)

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)

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

Function and class complexity

class TooManyLocalsViolation(node, text=None)

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 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)

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 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)

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.

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)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids putting too many expressions in a unit of code.

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)

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 attributes of a class.

Configuration:

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

New in version 0.1.0.

See also

https://en.wikipedia.org/wiki/God_object

error_template = 'Found too many methods: {0}'

code = 214

class TooManyBaseClassesViolation(node, text=None)

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.

See also

https://en.wikipedia.org/wiki/Composition_over_inheritance

error_template = 'Too many base classes: {0}'

code = 215

class TooManyDecoratorsViolation(node, text=None)

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 youself: 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

Structures complexity

class TooDeepNestingViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids nesting blocks too deep.

Reasoning:

If nesting is too deep that indicates usage of a 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)

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 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.

See also

https://github.com/Miserlou/JonesComplexity

error_template = 'Found line with high Jones Complexity: {0}'

code = 221

class TooManyConditionsViolation(node, text=None)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to have conditions with too many logical operators.

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)

Bases: wemake_python_styleguide.violations.base.ASTViolation

Forbids to use many elif branches.

Reasoning:

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

Solution:

There are different design patters 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)

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 a 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