Why my tests fail only during pre-commit ?

Symptom

Recently I ran across (what I thought was) strange behaviour.

I use pre-commit for all my git commits, and one of the step is to ensure that
all the unit tests pass.

I also have a make target to run just the unit tests.

Each time when I tried commit, the unit test step in the pre-commit
would fail - which indicates that the unit tests did not pass. But when I run
make unit it would pass. 🤷

During the pre-commit, pytest would return with exit code 5 - which
indicates no tests ran. So it wasn't that one or more tests failed during
pre-commit - but nothing was tested - and due to non-zero exit code,
pre-commit prevented the commit (as it should)

Root Cause

It turns out pre-commit runs all the steps only on the modified files.
It passes the list of modified files to each command - implicitly.
That is, unless you configure it not to. 😀

So when I modify a source file (not a test file) - pytest would "ignore"
the file - since it would not contain any tests.

Solution

Now that we understand the "why" - solution is obvious.

Patient : Doctor, it hurts when I do this

Doctor : Then don't do that
😂

For the unit test step, ask pre-commit not to pass the file names as an argument
via pass_filenames: false

Reference

• Stackoverflow