pytest-leela 0.1.0

Type-aware mutation testing for Python — fast, opinionated, pytest-native

pytest-leela

Type-aware mutation testing for Python.

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What it does

pytest-leela runs mutation testing inside your existing pytest session. It injects AST mutations via import hooks (no temp files), maps each mutation to only the tests that cover that line, and uses type annotations to skip mutations that can't possibly fail your tests.

It's opinionated: we target latest Python, favour speed over configurability, and integrate with pytest without separate config files or runners. If that fits your workflow, great. MIT licensed — fork it if it doesn't.

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Install

pip install pytest-leela

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Quick Start

Run mutation testing on your whole test suite:

pytest --leela

Target specific modules (pass --target multiple times):

pytest --leela --target myapp/models.py --target myapp/views.py

Only mutate lines changed vs a branch:

pytest --leela --diff main

Limit CPU cores:

pytest --leela --max-cores 4

Cap memory usage:

pytest --leela --max-memory 4096

Combine flags:

pytest --leela --diff main --max-cores 4 --max-memory 4096

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Features

• Type-aware mutation pruning — uses type annotations to skip mutations that can't possibly trip your tests (e.g. won't swap + to - on a str operand)
• Per-test coverage mapping — each mutant runs only the tests that exercise its lines, not the whole suite
• In-process execution via import hooks — mutations applied via sys.meta_path, zero filesystem writes, fast loop
• Git diff mode — --diff <ref> limits mutations to lines changed since that ref
• Framework-aware — clears Django URL caches between mutants so view reloads work correctly
• Resource limits — --max-cores N caps parallelism; --max-memory MB guards memory

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Requirements

• Python >= 3.12
• pytest >= 7.0

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License

MIT