Find source of PyTest warning

March 23, 2021

PyTest flips on DeprecationWarning and PendingDeprecationWarning as typically those running PyTest are a developer or advanced users. When the package uses warnings.warn to emit a DeprecationWarning, it can be hard to know from where in the tested package a warning is coming from.

To turn on a large amount of warnings, similar to what might be seen on CI:

python -Walways::DeprecationWarning -m pytest

If there is only one type of DeprecationWarning being omitted, a simple way to find the source of the warning is Python warning control -W. This will raise an exception at the warning with traceback:

python -Werror::DeprecationWarning -m pytest

However, often there are multiple DeprecationWarning emitted from different sources, and chances are the one of interest isn’t the first. Perhaps the warning comes from the Python distribution (e.g. Miniconda) itself. In that case, one can insert temporary warning trap into the test function or the user function:

warnings.filterwarnings("error", category=DeprecationWarning)

This might have to be done iteratively to get past the point where the uninteresting DeprecationWarning happen until you home in on the location of the DeprecationWarning of interest.