Last failed, failed first: --lf, --ff

Pytest always remembers tests that failed in previous sessions, and can reuse that information to skip right to the tests that have failed previously. This is excellent news if you are incrementally fixing a test suite after a large refactoring, as mentioned in the previous section.

You can run the tests that failed before by passing the --lf flag (meaning last failed):

λ pytest --lf tests/core
...
collected 6 items / 4 deselected
run-last-failure: rerun previous 2 failures

When used together with -x (--maxfail=1) these two flags are refactoring heaven:

λ pytest -x --lf 

This lets you start executing the full suite and then pytest stops at the first test that fails. You fix the code, and execute the same command line again. Pytest starts right at the failed test, and goes on if it passes (or stops again if you haven't yet managed to fix the code yet). It will then stop at the next failure. Rinse and repeat until all tests pass again.

Keep in mind that it doesn't matter if you execute another subset of tests in the middle of your refactoring; pytest always remembers which tests failed, regardless of the command-line executed.

If you have ever done a large refactoring and had to keep track of which tests were failing so that you didn't waste your time running the test suite over and over again, you will definitely appreciate this boost in your productivity.

Finally, the --ff flag is similar to --lf, but it will reorder your tests so the previous failures are run first, followed by the tests that passed or that were not run yet:

λ pytest -x --lf
======================== test session starts ========================
...
collected 6 items
run-last-failure: rerun previous 2 failures first