Tests take longer to run when coverage context is recorded, expected?

Perhaps you are already aware of this and it is expected, but at least for the purpose of documenting it:

Running tests while recording coverage context for who-tests-what (by using the .coveragerc mentioned below), takes considerably longer than usual. As a small example, running 300 unit tests used to take 15 seconds and with this it takes around 50 seconds. On bigger batches of tests it became quite impactful. Perhaps this is all expected.

The .coveragerc file is:

[run]
dynamic_context = test_function

As described in https://nedbatchelder.com/blog/201810/who_tests_what_is_here.html

The command was:

python coverage run -p --branch /path/py.test tests/unit/module/

Measured on:

Python==3.6.5
coverage==5.0a4

[nedbat]

I’ve made some more speed improvements on master. Can you try them?

[nedbat]

pytest-cov is a pytest plugin for coordinating coverage measurement during test runs. It’s faster for two reasons:

• It starts coverage after pytest has collected tests, etc, so those early phases of pytest are not subject to the usual coverage slow-down
• It uses internal pytest events to switch the dynamic context. Without that, coverage has to constantly check whether a new test has started.