pytest reproducer

When using the following together, pytest 7.4.0 creates multiple instances of modules instead of reusing already-executed ones. The bug triggers in pytest’s collection phase.

Necessary for the bug to trigger:

• --import-mode=importlib (I’m sure it’s also possible with append in another way)
• --doctest-modules
• no src/ layout
• importing from a submodule.

Run with e.g.

hatch run pytest
# or if you like typing
python -m virtualenv .venv; source .venv/bin/activate; pip install -e .; pytest

No matter if you opt to install in dev mode like this, specify PYTHONPATH, or install regularly, the bug is reproducible in any case.

how it works

The module in __init__.py gets created and executed twice, but when each instance executes from .singleton import MyVindictiveSingleton, that reuses the same instance of the submodule.

flowchart LR
A[__init__.py] -->|imports| C(singleton.py)
B[__init__.py] -->|imports| C(singleton.py)

Loading

tracebacks

The two tracebacks are almost identical, but show that a cached version of a submodule is used (fewer frozen importlib._bootstrap frames).

It’s also possible to put some code in conftest.py or a plugin and have the second execution there.

File "<venv>/lib/python3.11/site-packages/_pytest/pathlib.py", line 538, in import_path
  spec.loader.exec_module(mod)  # type: ignore[union-attr]
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "<frozen importlib._bootstrap_external>", line 940, in exec_module
File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
File "<proj>/pytest_doctest_import_twice/__init__.py", line 1, in <module>
  from .singleton import MyVindictiveSingleton
File "<frozen importlib._bootstrap>", line 1178, in _find_and_load
File "<frozen importlib._bootstrap>", line 1128, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
File "<frozen importlib._bootstrap>", line 1178, in _find_and_load
File "<frozen importlib._bootstrap>", line 1149, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 690, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 940, in exec_module
File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
File "<proj>/pytest_doctest_import_twice/__init__.py", line 3, in <module>
  instance = MyVindictiveSingleton()
             ^^^^^^^^^^^^^^^^^^^^^^^

File "<venv>/lib/python3.11/site-packages/_pytest/pathlib.py", line 538, in import_path
  spec.loader.exec_module(mod)  # type: ignore[union-attr]
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "<frozen importlib._bootstrap_external>", line 940, in exec_module
File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
File "<proj>/pytest_doctest_import_twice/__init__.py", line 3, in <module>
  instance = MyVindictiveSingleton()
             ^^^^^^^^^^^^^^^^^^^^^^^