I'm not sure I like the API feel after this change: two parameters that can't be used at the same time.

As Karthikeyan points out, this can be achieved using side_effect. Personally, I'd prefer a PR that adds a unit test showing that this approach works, and an example included in the docs as suggested above.

Also, be aware that there can be better options for mocking files. https://github.com/jmcgeheeiv/pyfakefs/ and https://docs.pytest.org/en/latest/tmpdir.html would be my recommendation.