flake8-no-pep420

A flake8 plugin to ban PEP-420 implicit namespace packages.

Requirements

Python 3.8 to 3.12 supported.

Installation

First, install with pip:

python -m pip install flake8-no-pep420

Second, if you define Flake8’s select setting, add the INP prefix to it. Otherwise, the plugin should be active by default.

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Rationale

Implicit namespace packages are directories of Python files without an __init__.py. They’re valid and importable, but they break many tools, such as:

• unittest test discovery (and by extension, Django’s test runner)
• Coverage.py
• Mypy without its --namespace-packages option
• pytest

In most cases, tools fail silently, which can lead to a false sense of security:

• Tests may look legitimate but never run
• Code may be untested but not appear in coverage statistics
• Types may never be checked

PEP-420’s algorithm is non-trivial which is probably why such tools haven’t (yet) implemented it.

Rules

INP001: File is part of an implicit namespace package. Add __init__.py?

flake8-no-pep420 will trigger this on the first line of any file that sits in a directory without an __init__.py file.

Often projects have a few root files not in packages, for which an __init__.py file should not be added. For example, Django projects normally have a manage.py file in the root of their repository. In these cases you can ignore the INP001 error. It’s possible to use # noqa: INP001 to ignore the error in-line, but this isn’t possible if the first line is a shebang, such as in Django’s manage.py. In such cases it’s preferable to use Flake8’s per-file-ignores option, for example in setup.cfg:

[flake8]
# ...
per-file-ignores =
    manage.py:INP001