assert-no-pytest-plugin-declarations

Assert that no conftest declares a pytest plugin.

Why

pytest loads a plugin once. Whether a given pytest_plugins line is doing anything is therefore decided by files other than the one it is written in — by a module in another directory, or by a conftest further up the path that has already asked for the same module. Reading the line tells you nothing about whether it is load-bearing, and no amount of reading the file around it will either.

A fixture has two ways to reach a suite that do not have that problem. It can be inherited, by sitting in a conftest above the suite that wants it. Or it can be imported, by a from ... import ... written in the conftest that needs it. Both are ordinary Python that ordinary tools follow: pylint reports the import nothing uses, mypy resolves it, and the sibling tool assert-pytest-fixture-name-is-needed reads the name it binds. A pytest_plugins line is a second mechanism for the same job that none of the three can see through.

It also holds package dependencies up, silently. A declaration that registers no fixture at all still imports its module, and so keeps that module's own imports load-bearing with nothing anywhere saying so. The line cannot be deleted without a reader first working out, across the whole tree, what would stop being imported if it went.

So the rule is the whole of it. Not a redeclaration, not a duplicate of one further up: any declaration at all.

Installation

pip install assert-no-pytest-plugin-declarations

Usage

# Every test tree this repository publishes
assert-no-pytest-plugin-declarations test

# The same, leaving out tests somebody else wrote
assert-no-pytest-plugin-declarations test \
  --exclude 'test/vendor/*'

Options

Exit codes

What counts as a declaration

Any assignment to the name pytest_plugins, whatever it is assigned:

The value is never inspected, because the declaration this tool most wants to refuse is the one whose value cannot be worked out by reading: a name built elsewhere, or a list a loop appends to. An empty list is a finding like any other. It registers nothing today and is an invitation to the next person to add an entry rather than a conftest.

What is walked

A directory argument is read recursively for *.py files. .git, __pycache__ and node_modules are skipped, and everything else you want left out goes in --exclude. Each file is read on its own and needs nothing from any other, so a single file is as valid an argument as a tree.

Every Python file is read, not only files named conftest.py. pytest honours the name in a conftest and in a test module, and a line that sits anywhere else is one that starts working the day the file is renamed or its contents are moved.

GitHub Actions

- name: Assert no conftest declares a pytest plugin
  uses: 10U-Labs/assert-no-pytest-plugin-declarations@latest
  with:
    trees: test

annotate defaults to true there, so each finding lands on the line it names in the diff.

License

Apache-2.0