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.
Install & Compatibility
Where this runs
tested against v0.7.0 · pip install
no network on importno background threads
Install × environment matrix
Each cell = how many times install + import succeeded across repeated harness runs. Partial = flaky.
glibc = Debian/Ubuntu slim · musl = Alpine Linux
muslpy 3.10–3.950 runs
installs and imports cleanly · install 0.0s · import 0.020s · 19.2MB
glibcpy 3.10–3.950 runs
installs and imports cleanly · install 1.7s · import 0.020s · 20MB
17MB installed
● package 17MB
Code
Verified usage
Verified import paths — ran on the pinned version, not inferred.
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.
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
Debug
Known issues
breakingVersion 0.7.0 and later dropped support for Python versions older than 3.6.fixEnsure your project runs on Python 3.6 or newer when using mccabe 0.7.0 or later.
affects: 0.7.0+
gotchaWhen used with flake8, the mccabe plugin is disabled by default. You must explicitly enable it by setting the maximum complexity threshold.fixUse 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`.
affects: All versions with flake8 integration
gotchaTo 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.fixPlace `# noqa: C901` directly on the line where the `def` keyword or the decorator above it appears for the function you want to ignore.
affects: All versions with flake8 integration
Audit
Dependencies
flake8requiredMcCabe primarily functions as a plugin for flake8, integrating its complexity checks into the broader linting workflow.