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tarpas/pytest-testmon

A pytest plugin that re-runs only the tests affected by your recent code changes, with optional continuous watch mode.

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What is it?

pytest-testmon is a pytest plugin that records which code each test depends on and uses that dependency database to pick only the tests affected by recent changes. The first run with --testmon builds the database in .testmondata; later runs execute just the affected tests, which shortens feedback loops on large suites.

The project addresses the difficulty of determining affected tests in a dynamic language like Python, where static import graphs are not enough. Its documentation describes how it tracks dependencies and how reliable the selection is. Combined with pytest-watch, it can act as a continuous test runner that reruns the right tests on every save.

The package is installed with pip and enabled with a single --testmon flag. The project also runs an opt-in program in which the maintainers set up testmon in open-source projects' CI.

Key features

  • 01Records dependencies between tests and code in a local .testmondata database.
  • 02Re-runs only the tests affected by recent file changes.
  • 03Works as a pytest plugin enabled with the --testmon flag.
  • 04Pairs with pytest-watch for continuous test running on save.

Use cases

  • Speeding up local test runs in a large Python project with a slow pytest suite.
  • Running a fast subset of tests during active development.
  • Setting up continuous testing that reruns affected tests on each change.

Quick start

  1. 1

    Install the plugin with pip

    pip install pytest-testmon
  2. 2

    Build the dependency database (saved to .testmondata)

    pytest --testmon
  3. 3

    Change some of your code (with test coverage)

  4. 4

    Run only the tests affected by recent changes

    pytest --testmon

Things to know

  • Selection relies on dependency tracking, so reliability depends on how the code is structured.

In collections

Testing frameworks & test automation tools

Topics

#pytest#testing#python#test-selection#continuous-testing#ci#build-tool#test-automation

FAQ

What is pytest-testmon?
A pytest plugin that re-runs only the tests affected by your recent code changes, with optional continuous watch mode. pytest-testmon is a pytest plugin that records which code each test depends on and uses that dependency database to pick only the tests affected by recent changes. The first run with --testmon builds the database in .testmondata; later runs execute just the affected tests, which shortens feedback loops on large suites.
Is pytest-testmon free to use? What license does it have?
pytest-testmon is open source under the MIT license. Check the license terms before using it in commercial products.
Is pytest-testmon still maintained?
Actively maintained. The last commit to pytest-testmon was on Nov 2, 2026.
What can I use pytest-testmon for?
Speeding up local test runs in a large Python project with a slow pytest suite. Running a fast subset of tests during active development. Setting up continuous testing that reruns affected tests on each change.
How do I install pytest-testmon?
Install the plugin with pip: pip install pytest-testmon Build the dependency database (saved to .testmondata): pytest --testmon Change some of your code (with test coverage) Run only the tests affected by recent changes: pytest --testmon
Where can I get pytest-testmon?
The source code and installation instructions are on GitHub: https://github.com/tarpas/pytest-testmon

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