take TDD to a new level with py.test and testmon

pip install pytest-testmon==0.9.3


Build Status

This is a py.test plug-in which automatically selects and re-executes only tests affected by recent changes. How is this possible in dynamic language like Python and how reliable is it? Read here: Determining affected tests


pip install pytest-testmon

# build the dependency database and save it to .testmondata
py.test --testmon

# change some of your code (with test coverage)

# only run tests affected by recent changes
py.test --testmon

# start from scratch (if needed)
rm .testmondata

# automatic re-execution on every file change with pytest-watch (https://github.com/joeyespo/pytest-watch)
pip install pytest-watch
ptw -- --testmon

Other switches

--project-directory= only files in under this directory will be tracked by coveragepy. Default is rootdir, can be repeated


Add testmon to the pytest.ini

# If you want to separate different environments running the same sources.
run_variant_expression = os.environ.get('DJANGO_SETTINGS_MODULE') + ':python' + str(sys.version_info[:2])
addopts = --testmon # you can make --testmon a default if you want

More complex run_variant_expression can be written: the os, sys and hashlib modules are available, and there is a helper function md5(s) that will return hashlib.md5(s.encode()).hexdigest().

Configuring subprocess tracking

If your test suite uses subprocesses testmon supports this. You just have to configure python+coverage so that the coverage hook is executed with every python process started. You can do this by installing coverage_pth

pip install coverage_pth

If there is any problem you can still configure your python manually.

Troubleshooting - usual problems

Testmon selects too many tests for execution: Depending you your change it most likely is by design. If you changed a method parameter name, you effectively changed the whole hierarchy parameter -> method -> class -> module, so any test using anything from that module will be re-executed.

Tests are failing when running under testmon: It's quite unlikely testmon influenced the execution of the test itself. However set of deselected and executed tests with testmon is highly variable, which means testmon is likely to expose undesired test dependencies. Please fix your test suite.

You can also try if your test is influenced under pytest-cov (coverage) without testmon. For reporting a bug a repository with description of unexpected behavior is the best, but please don't hesitate to report even if your project is closed source. We'll try to fix it!


testmon is approaching completeness. Unfortunately the classic console UI is reaching it's usability limits even without testmon. With testmon it's even a little more difficult to determine which tests are being executed, which are failing and why. Next step would be an implementation or integration of GUI. I don't like any of the existing graphical test runners, so if you have some better new concept in mind, get in touch!


Individual test outcomes depend on many things, so let's write a little about some of them.

  1. executed python code inside the tested project (which presumably changes very frequently, little by little)
  2. environment variables (e.g. DJANGO_SETTINGS_MODULE), python version (the run_variant_expression config value denotes these)
  3. executed python code in all of the libraries (which presumably change infrequently)
  4. data files (txt, xml, other project assets)
  5. external services (reached through network)

testmon so far deals with incrementally running tests when faced with the 1. and 2. category of changes.

Later versions can implement some detection of other categories

libraries: we could compare pip freeze between runs, but it's slow

data files: Probably the best bet here is a configuration where the developer would specify which files does a test depend on


Big thanks to Qvantel, Nick Coghlan , Abilian SAS and Infinit for being silver sponsors of the first release of testmon. List of all contributors to our campaign is here . Thanks a lot to all contributors.