pytest-leela 0.8.0

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

When no --target is given, pytest-leela auto-discovers source files:

1. Looks for a target/ or src/ directory (in that order) and mutates all .py files inside it
2. If neither exists, falls back to the project root (skipping .venv, build, dist, __pycache__, node_modules, .git, and similar non-source directories)

Test files (test_*.py, *_test.py, conftest.py, tests.py) are always excluded from mutation.

Exclude patterns from [tool.pytest-leela] (see Configuration) are applied after target discovery, so you can auto-discover src/ while still excluding specific paths.

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

Generate an interactive HTML report:

pytest --leela --leela-html report.html

Benchmark optimization layers:

pytest --leela-benchmark

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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
• HTML report — --leela-html generates an interactive single-file report with source viewer, survivor navigation, and test source overlay
• CI exit codes — exits non-zero when mutants survive, so CI pipelines fail on incomplete kill rates
• Benchmark mode — --leela-benchmark measures the speedup from each optimization layer

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Configuration

Configure pytest-leela in your pyproject.toml under [tool.pytest-leela]:

[tool.pytest-leela]
exclude = [
    "*/migrations/*",
    "manage.py",
    "*/conftest.py",
]
operators = [
    "arithmetic",
    "comparison",
    "boolean",
    "return",
]

exclude

A list of glob patterns. Files whose path (relative to the project root) matches any pattern are excluded from mutation. Uses fnmatch matching — * matches within a single directory, */migrations/* matches any migrations directory at any depth.

Default: [] (nothing excluded).

operators

A list of operator category names controlling which mutation types are applied. Use "all" as a shorthand to enable every category.

Default: ["arithmetic", "comparison", "boolean", "unary", "return"]

Category	What it mutates	Default
arithmetic	+ - * / // % **	on
comparison	== != < <= > >= is in and negations	on
boolean	and / or	on
unary	unary - + not	on
return	return values (True/False, None, literals, expressions)	on
bitwise	& | ^ << >>	off
augmented_assign	+= -= *= etc.	off
ternary	x if cond else y branch swaps	off
control_flow	break / continue swaps	off
exception	except handler broadening, body-to-raise	off

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Pragma Skip

Add # leela: skip to suppress mutations on individual lines:

result = x + y  # leela: skip

To skip an entire file, place the pragma on line 1:

# leela: skip
"""This module is excluded from mutation testing."""

def legacy_code():
    ...

The pragma is detected via Python's tokenize module, so it only matches actual comments — not string literals or docstrings that happen to contain the text.

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HTML Report

--leela-html report.html generates a single self-contained HTML file with no external dependencies.

What it shows:

• Overall mutation score badge
• Per-file breakdown with kill/survive/timeout counts
• Source code viewer with syntax highlighting

Interactive features:

• Click any line to see mutant details (original → mutated code, status, relevant tests)
• Survivor navigation overlay — keyboard shortcuts: n next survivor, p previous, l list all, Esc close
• Test source overlay — click any test name to see its source code

Uses the Catppuccin Mocha dark theme.

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Requirements

• Python >= 3.12
• pytest >= 7.0

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License

MIT