McCabe Code Complexity Checker

Common errors

• C901 'function_name' is too complex (X)

  cause The McCabe cyclomatic complexity of the function or method exceeds the configured maximum threshold, indicating potentially overly complex code that might be hard to understand or test.
  fix Refactor the function to reduce its complexity, or explicitly set a higher --max-complexity threshold when running flake8, or add `# noqa: C901` to the line defining the function to ignore the warning for that specific function. For example, `flake8 --max-complexity 10 your_project/`

• flake8: error: no such option: --max-complexity

  cause This error occurs when the `flake8` command is run with the `--max-complexity` option, but either the `mccabe` plugin is not installed or `flake8` is an older version that does not recognize this option, or `mccabe` is not properly integrated with `flake8`.
  fix Ensure both `flake8` and `mccabe` are installed and up-to-date (`pip install --upgrade flake8 mccabe`). If the problem persists, try reinstalling them: `pip uninstall flake8 mccabe; pip install flake8 mccabe`.

• ModuleNotFoundError: No module named 'mccabe'

  cause The `mccabe` package is not installed in the Python environment being used, or the Python interpreter cannot find it in its configured paths.
  fix Install the `mccabe` package using pip: `pip install mccabe`. If using a virtual environment, ensure it is activated before installation.

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