Question: Why does this import work by default, but not with --import-mode=importlib

[LuminairPrime]

I was testing --import-mode=importlib because I read that it will eventually be the default mode.

I have test_x.py with:

import y
def test_x(): assert 1

And y.py with:

print("test?")

The output with a default pytest is a pass:

C:\pytest_experiment>pytest
========================================================== test session starts ==========================================================
platform win32 -- Python 3.10.1, pytest-7.1.1, pluggy-1.0.0
rootdir: C:\pytest_experiment
plugins: forked-1.4.0, monkeytype-1.1.0, xdist-2.5.0
collected 1 item

test_x.py .                                                                                                                        [100%]

=========================================================== 1 passed in 0.02s ===========================================================

And the output with a pytest --import-mode=importlib is a failed import:

C:\pytest_experiment>pytest --import-mode=importlib
========================================================== test session starts ==========================================================
platform win32 -- Python 3.10.1, pytest-7.1.1, pluggy-1.0.0
rootdir: C:\pytest_experiment
plugins: forked-1.4.0, monkeytype-1.1.0, xdist-2.5.0
collected 0 items / 1 error

================================================================ ERRORS =================================================================
______________________________________________________ ERROR collecting test_x.py _______________________________________________________
ImportError while importing test module 'C:\pytest_experiment\test_x.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
test_x.py:1: in <module>
    import y
E   ModuleNotFoundError: No module named 'y'
======================================================== short test summary info ========================================================
ERROR test_x.py
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
=========================================================== 1 error in 0.11s ============================================================

What's up? Is that a bug, or do I need to do something else with --import-mode=importlib ?

[nicoddemus]

Hi @LuminairPrime,

I was testing --import-mode=importlib because I read that it will eventually be the default mode.

Just a note: I don't think that will be the case, because unfortunately it has its own issues as well.

What's up?

pytest needs to import test_x.py in order to collect and run the tests:

1. The default mode (append), will append the directory that contains test_x.py to sys.path and then call __import__.
2. The importlib mode uses a separate mechanism which doesn't require pytest to change sys.path.

The append mode works in that case because sys.path is updated to import test_x.py, which causes y to be importable.

[LuminairPrime]

Hmmm. In this case, the path that import-mode=importlib needs to find is equal to 1) the directory where pytest is run, and 2) the test file directory, and 3) the tested file directory. Shouldn't every import mode succeed at this?

Where do I need to place y.py for import-mode=importlib to find it?

[nicoddemus]

In this case, the path that import-mode=importlib needs to find is equal to 1) the directory where pytest is run, and 2) the test file directory, and 3) the tested file directory. Shouldn't every import mode succeed at this?

1. Not really, just running pytest by itself doesn't change the sys.path. What changed the sys.path to the current directory was the fact that test_x.py was in the current directory, and --import-mode=append changes sys.path to import test files.
2. Correct.
3. Hmm this seems to be the same as 2, unless I'm misunderstanding.

Indeed import modes and test discovery is a somewhat complicated topic (because Python's import is complicated). We have a dedicated section in the docs dedicated to this topic: https://docs.pytest.org/en/7.1.x/explanation/pythonpath.html

Where do I need to place y.py for import-mode=importlib to find it?

Usually the recommendation is using a virtual environment and installing your application there.

For quick one-off scripts or very simple projects, you can run python -m pytest instead of pytest, because the former adds the current directory to sys.path (that is normal Python behavior when executing python -m, not being specific to pytest).

[LuminairPrime]

For my personal use the default behavior of pytest works, and I can apply sys.path.append for extra distant directories. What I'm trying to get to the bottom of here is whether import-mode=importlib is broken. To ME it seems like it should work in this case.