Cohesion

flake8-cohesion is a flake8 extension for measuring Python class cohesion.

In computer programming, cohesion refers to the degree to which the elements of a module belong together. Thus, cohesion measures the strength of relationship between pieces of functionality within a given module. For example, in highly cohesive systems functionality is strongly related.

• Wikipedia

When cohesion is high, it means that the methods and variables of the class are co-dependent and hang together as a logical whole.

• Clean Code pg. 140

Some of the advantages of high cohesion, also by Wikipedia:

• Reduced module complexity (they are simpler, having fewer operations).
• Increased system maintainability, because logical changes in the domain affect fewer modules, and because changes in one module require fewer changes in other modules.
• Increased module reusability, because application developers will find the component they need more easily among the cohesive set of operations provided by the module.

Differences to the cohesion package

flake8-cohesion is a fork of cohesion. However, because it seems abandoned, I decided to create a new package and change the calculations to match a more regular workflow when coding in python:

• The following method types are not considered when calculating class cohesion:

  • abstractmethods
  • classmethods
  • staticmethods
  • properties

• Methods containing only a pass statement are not considered.

• Class variables that are defined in class definition are not considered when they are not accessed in any considered method.

Installation

Install with pip:

pip install flake8-cohesion

requires python >= 3.9

Usage

Check with flake8:

python -m flake8

Violations

flake8-cohesion currently reports only one violation:

Options

flake8-cohesion supports two options:

example flake8 configuration file:

[flake8]
cohesion-below = 70.0
cohesion-strict = true