Mocking import_module() function causes all subsequent mocks to fail silently

[miguelgrinberg]

Bug report

This is a 3.11 regression. Here is the simplest example I could come up with to demonstrate the issue:

from unittest import mock

def f():
    return 'not mocked'

with mock.patch('importlib.import_module') as import_module:
    with mock.patch('__main__.f', return_value='mocked'):
        print(f())

Here is the output under several Python versions:

$ python3.8 mocktest.py
mocked
$ python3.9 mocktest.py
mocked
$ python3.10 mocktest.py
mocked
$ python3.11 mocktest.py
not mocked

In case it helps, I've been looking through the commit list for mock.py and found that ab7fcc8 is the first change that shows the broken behavior.

Your environment

I'm using CPython 3.11.0 on Intel Mac.

$ python3.11
Python 3.11.0 (main, Oct 26 2022, 10:08:00) [Clang 14.0.0 (clang-1400.0.29.102)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

[ronaldoussoren]

I'm not sure if this is a bug, you're mocking importlib.import_module and then do something where a modern implementation would (and as of 3.11 does) use that function to find the module where name resolution should start. The code prints "not mocked" because mock.patch indirectly uses importlib.import_module and hence calls the mock object instead of the real implementation.

This worked in older versions of Python because those used a custom implementation of this functionality instead of relying on a public API for this.

[brettcannon]

I agree this isn't a bug for the reasons Ronald outlined.

Thanks for at least reporting this, @miguelgrinberg !