How to mock methods like `pathlib.Path.is_dir`?

I used to have some testing code for mocking a simple directory structure when working with pathlib. I recently tried to run these tests to learn that some of the internals in pathlib have changed in recent Python versions, breaking my mocking setup. I managed to resolve some of the issues already, but I got stuck on an issue that feels like it shouldn't be one.

Concretely, I have been mocking pathlib.Path.is_dir as follows:

from pathlib import Path
from unittest import mock

some_path = Path("/some/path/")
with mock.patch("pathlib.Path.is_dir") as m_is_dir:
    m_is_dir.side_effect = lambda p: p.name == ""
    print(Path.is_dir(some_path))  # this is just a placeholder

Now, the problem is that the code in newer Python versions (internally) seems to be using some_path.is_dir() instead. As a result, I get a TypeError about a missing positional argument p for my lambda expression. This can be simulated by the following code

with mock.patch("pathlib.Path.is_dir") as m_is_dir:
    m_is_dir.side_effect = lambda p: p.name == ""
    print(some_path.is_dir())  # this is just a placeholder

This can be fixed by rewriting the mocking function

with mock.patch("pathlib.Path.is_dir") as m_is_dir:
    m_is_dir.side_effect = lambda: some_path.name == ""
    print(some_path.is_dir())  # this is just a placeholder 

but of course that breaks the tests for the older versions again.

I have tried to patch some_path.is_dir directly, using

with mock.patch("pathlib.Path.is_dir") as m_is_dir:
    m_is_dir.side_effect = lambda p: p.name == ""
    some_path.is_dir = lambda: m_is_dir(some_path)
    print(some_path.is_dir())  # this is just a placeholder 

but that resulted in an AttributeError because is_dir is read-only.

I actually would have assumed that patching Path.is_dir(p) (indirectly) patch the bound method p.is_dir as well, but that does not seem to be the case. So that leaves me with the question: is there a robust way to patch methods that can be called either way? Or more concretely: how can I patch Path.is_dir so that the following works?

from pathlib import Path
from unittest import mock

some_path = Path("/some/path/")
with mock.patch(???) as m_is_dir:
    m_is_dir.side_effect = lambda p: p.name == ""
    print(Path.is_dir(some_path))
    print(some_path.is_dir())

It might be that there is an obvious solution, but I am in too deep and I don't seem to be able to find it, so any help would be greatly appreciated.

posted about 1 month ago

CC BY-SA 4.0

mr Tsjolder‭

518 reputation 6 14 61 6