How to run tests of python library without compiling the whole library

I’m packaging a library, and some test fails. I’d like to run the tests manually to see what is the problem, but unfortunately I can’t find how to run the test phase without recompiling the whole library. For now, I’m simply doing, in the nixpkgs folder:

nix-build -A python38.mylibrary

Is there a way to run only the tests and cache the compilation step? Is it possible to have access to a shell, and be left right before the test runs? I tried to do:

nix-shell -A python38.mylibrary
buildPhase

but then when I try to run the test phase I get lot’s of errors like E NameError: name '__IPYTHON__' is not defined or AttributeError: partially initialized module 'mylibrary' has no attribute 'settings' (most likely due to a circular import) which I usually don’t have on regular tests.

Thanks!

Try breakpointHook. It will pause the build at the failure point, and allow you to shell into the build environment to debug it.