Still getting ImportPathMismatchError when using tox and fixtures conftest.py

[astrojuanlu]

I am observing a weird situation that makes me question my pytest setup. So far my tests were working well, but today I decided to add some fixtures to conftest.py that actually import the library code, and started getting these errors when running tox:

_________________________________________________________________________________________ ERROR collecting src/poliastro/core/thrust/change_argp.py __________________________________________________________________________________________
.tox/py3-fast/lib/python3.8/site-packages/pytest_doctestplus/plugin.py:246: in collect                                                                                                                                                        
    module = import_path(fspath, root=self.config.rootpath)                                                                                                                                                                                   
.tox/py3-fast/lib/python3.8/site-packages/_pytest/pathlib.py:556: in import_path                                                                                                                                                              
    raise ImportPathMismatchError(module_name, module_file, path)                                                                                                                                                                             
E   _pytest.pathlib.ImportPathMismatchError: ('poliastro.core.thrust.change_argp', '/home/juanlu/Projects/LSF/poliastro/library/.tox/py3-fast/lib/python3.8/site-packages/poliastro/core/thrust/change_argp.py', PosixPath('/home/juanlu/Projects/LSF/poliastro/library/src/poliastro/core/thrust/change_argp.py')) 

There are no __init__.py files under my tests directory, but I acknowledge that maybe it's a weird interaction between tox, pytest, and pytest-doctestplus. The error disappears if I remove my library imports from conftest.py.

This is probably similar to what @matthewfeickert observed in scikit-hep/pyhf#1467 and I solved it in the same way: removing src from testpaths. I'm aware there's also PY_IGNORE_IMPORTMISMATCH=1, but it feels kind of hacky.

Basically, my question is: is this a bug? A feature? Am I doing anything wrong by removing src from testpaths (like suddenly disabling doctests without noticing)? Is PY_IGNORE_IMPORTMISMATCH=1 the only way?

[portyu9]

Hi @astrojuanlu

The mismatch is real: pytest is ending up with two different copies of poliastro under the same import name.

Your conftest.py imports the installed copy from the tox environment, while collecting src for doctests causes pytest/pytest-doctestplus to encounter the source-tree copy. Once one copy has already been imported, pytest detects that the module being collected comes from a different path and raises ImportPathMismatchError.

I would not use PY_IGNORE_IMPORTMISMATCH=1 as the normal solution. It disables the safety check and can hide the fact that you are testing/importing a different copy than intended.

The preferred fix is to make each pytest invocation use one package location consistently:

• If you want to test the installed tox package, don't simultaneously collect the source tree as the same importable package.
• If you specifically need source-tree doctests, run those in an invocation/environment where the source tree is the copy being imported.
• For modern pytest projects, --import-mode=importlib is also preferable to the older prepend import mode because it avoids many sys.path/module-name collisions, although a plugin which explicitly imports the installed package can still create the same fundamental two-copy problem.

So removing src from testpaths appears to "fix" it because it removes the second collection source, but the underlying issue is the simultaneous installed/source-tree imports.

Please mark this answer as accepted if it helped, thank you.