Behavior change in test discovery for `""` with pytest 9

[TomAugspurger]

pytest 9 changed how the empty string "" is interpreted when invoking pytest with multiple test files / directories.

This is almost surely caused by fixing #12083.

Given a layout like

test_root.py
a/
  test_a.py

with contents

# file: test_root.py
1/0

and

# file: a/test_a.py
def test_foo():
    pass

here's the output with pytest<9

❯ pytest -q '' a/
.                                                                                                                                                                                                                                       [100%]
1 passed in 0.14s

and with pytest==9.0.0

❯ pytest -q '' a/                                                                                                                                                                                                                       (base) 

=================================================================================================================== ERRORS ====================================================================================================================
_____________________________________________________________________________________________________ ERROR collecting debug/test_root.py _____________________________________________________________________________________________________
test_root.py:1: in <module>
    1/0
E   ZeroDivisionError: division by zero
=========================================================================================================== short test summary info ===========================================================================================================
ERROR test_root.py - ZeroDivisionError: division by zero
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
1 error in 0.29s

My main question is how '' specifically should be interpreted by pytest. Should it be the current directory? Or error?

But overall I think pytest's new behavior is much improved, and we were just being a bit sloppy with how we called pytest. Perhaps we were just relying on undefined behavior previously, in which case closing this as "won't fix" sounds good to me.

xref rapidsai/cudf#20570

---

Version info:

❯ uv pip list                                                                                                                                                                                                                           (base) 
Package   Version
--------- -------
iniconfig 2.3.0
packaging 25.0
pluggy    1.6.0
pygments  2.19.2
pytest    8.4.2

pytest: 8.4.2 / 9.0.0
OS: linux

[bluetech]

Thanks for the report, I will look into it hopefully soon.

[bluetech]

My main question is how '' specifically should be interpreted by pytest. Should it be the current directory? Or error?

It is currently interpreted as the current directory. Possibly it would be better to error, but that might break existing setups, I'm not sure it's worth it.

This is almost surely caused by fixing #12083.

Indeed. Given that '' is interpreted as ., the previous behavior then running pytest . a/ only included a/ was basically the bug #12083. So unfortunately you ended up inadvertently relying on this buggy pytest behavior. But we really needed to fix it.

At this point I don't think we will change the behavior again, so it'd be great if you can avoid the '' on your side.

[TomAugspurger]

Sounds good to me, thanks.