pytest-gremlins

Fast-first mutation testing for pytest. Speed that makes mutation testing practical for everyday TDD.

Let the gremlins loose. See which ones survive.

Key Features

• Speed-First Architecture - Mutation switching eliminates file I/O and module reloads. Run gremlins in seconds, not hours.
• Native pytest Integration - Zero configuration to start. Just add --gremlins to your pytest command.
• Coverage-Guided Selection - Only runs tests that actually cover the mutated code. 10-100x fewer test executions in well-modularized codebases.
• Incremental Caching - Results cached by content hash. Unchanged code skips re-testing entirely.
• Parallel Execution - Distribute gremlins across CPU cores for linear speedup.

Quick Start

# Install
pip install pytest-gremlins

# Run mutation testing
pytest --gremlins

That's it. pytest-gremlins will instrument your code, release the gremlins, and report which ones your tests zapped (good!) and which survived (test gaps!).

Why pytest-gremlins?

Code coverage lies. It tells you what code your tests execute, but not whether your tests would catch bugs.

Mutation testing answers a harder question: If I introduce a bug, will my tests fail?

The Problem with Existing Tools

See the full comparison guide for detailed, fair comparisons.

Our Solution: Speed Through Architecture

pytest-gremlins is fast because of how it works, not just parallelization:

1. Mutation Switching - Instrument once, toggle mutations via environment variable
2. Coverage Guidance - Only run tests that cover the mutated code
3. Incremental Analysis - Skip unchanged code on repeat runs
4. Parallel Execution - Safe parallelization with no shared state

Performance

Benchmarked against mutmut on a synthetic project:

Key findings:

• Sequential mode is slower due to subprocess isolation overhead; detailed profiling shows 1.7x slower on small targets
• Parallel mode delivers 3.73x speedup over mutmut
• With caching, subsequent runs are 13.82x faster
• pytest-gremlins found 117 mutations vs mutmut's 86, with 98% kill rate vs 86%

Example Output

================== pytest-gremlins mutation report ==================

Zapped: 142 gremlins (85%)
Survived: 18 gremlins (11%)
Timeout: 5 gremlins (3%)
Error: 2 gremlins (1%)

Top surviving gremlins:
  src/auth.py:42                   >= to >               (comparison)
  src/utils.py:17                  + to -                (arithmetic)
  src/api.py:88                    True to False         (boolean)

Run with --gremlin-report=html for detailed report.
=====================================================================

Timeout, Error, and Pardoned categories are only shown when their count is greater than zero.

Installation

# With pip
pip install pytest-gremlins

# With uv
uv add pytest-gremlins

# With poetry
poetry add pytest-gremlins

Requires Python 3.11+

Configuration

Zero configuration required for most projects. The plugin auto-discovers source paths from your pyproject.toml setuptools config (e.g., [tool.setuptools.packages.find]). If auto-discovery doesn't find your code, configure paths explicitly:

[tool.pytest-gremlins]
# Operators to use (default: all 5)
operators = ["comparison", "arithmetic", "boolean", "boundary", "return"]

# Paths to mutate (optional -- auto-discovered from setuptools metadata)
paths = ["src"]

# Patterns to exclude
exclude = ["**/migrations/*"]

# Report formats: "console", "html", "json" (default: console)
report = ["console", "html"]

# Enable incremental caching (default: false)
cache = true

# Fail if pardoned gremlins exceed 5% of total
max-pardons-pct = 5.0

The Gremlins Theme

We use Gremlins movie references as our domain language:

Documentation

Full documentation: pytest-gremlins.readthedocs.io

• Getting Started
• Configuration
• API Reference

Related Projects

• pytest-test-categories - Enforce Google test size standards in Python
• dioxide - Rust-backed dependency injection for Python

Contributing

Contributions welcome! See our Contributing Guide.

This project uses strict TDD discipline with BDD/Gherkin scenarios. All contributions must include tests written before implementation.

Note on code coverage: We target 69% coverage due to inherent limitations in measuring pytest plugins (import timing, subprocess execution). See CONTRIBUTING.md for details.

License

MIT License. See LICENSE.

Changelog

See CHANGELOG.md for release history.