Replacing tmp_path with FakePath

[jfindlay]

I am not sure if this is the most idiomatic way to do this or if there is a better way. I am replacing tmp_path cases with the following helper function:

def fake_path(path: Path | str) -> FakePath:
    """
    Get a pyfakefs fake `Path` class.
    """

    fake_pathlib_module = FakePathlibModule(FakeFilesystem())  # type: ignore[no-untyped-call]
    return fake_pathlib_module.Path(path)  # type: ignore[no-untyped-call]

For example:

-def test_case(tmp_path:Path) -> None:
+def test_case(fake_path:FakePath) -> None:
     with pytest.raises(FileNotFoundError, match=r"No such file or directory: '.*not/found'"):
-        assert tmp_path / "not/found"
+        assert fake_path("not/found")

Questions

1. Are there cases for which tmp_path works where the above fake_path function could not cover, like functionality that tmp_path provides that pyfakefs hasn't/won't implement?
2. Why is the implementation of fake_path so unintuitive? Could not pyfakefs provide fake_path as a fixture directly so the user doesn't have to instantiate FakeFilesystem and FakePathlibModule directly? Are there implementation or compatibility restrictions between pyfakefs and pytest that means it's better for the user to do this or be aware of it?
3. Why are the calls in fake_path untyped? Is typing not fully implemented in pyfakefs (or do I need to include the typeshed)?

[mrbean-bremen]

While it is possible to instantiate and use the fake fs classes directly (this was the original use case at Google when the library was first written, and partly still is today), you shouldn't need to do this if you use pytest with the fs fixture, unittest with fake_filesystem_unittest.TestCase, or fake_filesystem_unittest.Patcher directly. In these cases, it is sufficient to use just the standard filesystem functions, and, if wanted, some of the convenience functions.

Your specific use case (replacement of tmp_path) can be achieved by just using tempfile:

@pytest.fixture
def temp_path(fs):
    tmp_dir = tempfile.TemporaryDirectory()
    yield Path(tmp_dir.name)

This would return a newly created fake directory, which will be removed after the test. This should be completely sufficient to replace tmp_dir.
As the fake filesystem is created anew in each test, you could also use a fixed path and create and delete it manually:

@pytest.fixture
def temp_path(fs):
    tmp_path = Path("/tmp/foo")
    tmp_path.mkdir()
    yield tmp_path
    tmp_path.rmdir()  # this is not strictly needed, as the fake fs is removed after the test

As for question 3 (typing): as in most use cases these classes should not be needed, and typing can be a bit tricky given the changes between the Python versions, this is currently not done. Maybe I'll add it some time later...