McCabe Code Complexity Checker

0.7.0 · active · verified Sat Mar 28

McCabe is a Python library that provides a plugin for flake8, the Python code checker. It's used to analyze the cyclomatic complexity (McCabe complexity) of functions and methods in Python code, helping developers identify potentially over-complex code that might be difficult to understand, test, or maintain. The current version is 0.7.0. Its release cadence is slow, often aligning with updates to supported Python versions.

Warnings

breaking Version 0.7.0 and later dropped support for Python versions older than 3.6.
Fix: Ensure your project runs on Python 3.6 or newer when using mccabe 0.7.0 or later.
gotcha When used with flake8, the mccabe plugin is disabled by default. You must explicitly enable it by setting the maximum complexity threshold.
Fix: Use the `--max-complexity N` flag when running `flake8` (e.g., `flake8 --max-complexity 10`). Alternatively, configure it in a `setup.cfg`, `.flake8`, or `tox.ini` file under the `[flake8]` section, e.g., `max-complexity = 10`.
gotcha To ignore a specific McCabe complexity violation (C901) for a function, the `# noqa: C901` comment must be placed on the function definition line, not just any line within the function.
Fix: Place `# noqa: C901` directly on the line where the `def` keyword or the decorator above it appears for the function you want to ignore.

Install

pip install mccabe flake8 Install with Flake8
pip install mccabe Install Standalone

Imports

McCabeChecker
```
from mccabe import McCabeChecker
```
While this is the internal entry point for flake8 to discover the plugin, end-users typically do not import McCabeChecker directly. Its functionality is accessed via the `flake8` command-line tool.

Quickstart

McCabe is most commonly used as a plugin for `flake8`. After installing both `mccabe` and `flake8`, you enable McCabe checks by running `flake8` with the `--max-complexity` option, specifying the threshold at which a warning (C901) should be emitted. Alternatively, it can be run as a standalone script using `python -m mccabe`.

# To analyze a file using mccabe via flake8:
# 1. Install both libraries
# pip install mccabe flake8

# 2. Run flake8 with the --max-complexity flag
# (e.g., to flag functions with complexity > 10)
# flake8 --max-complexity 10 your_module.py

# Example of a simple Python file to check:
# your_module.py
def high_complexity_function(a, b):
    if a > 0:
        if b > 0:
            print('Both positive')
        else:
            print('A positive, B not')
    elif a < 0:
        if b < 0:
            print('Both negative')
        else:
            print('A negative, B not')
    else:
        print('A is zero')

# To use mccabe as a standalone script (less common):
# python -m mccabe --min 5 your_module.py

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