How to mock pathlib.Path.read_text for a particular file

Question

The unittest docs show you how to use mock_open to mock direct calls to builtins.open.

But how about mocking pathlib's read_text like this:

import pathlib

pathlib.Path("/path/to/file").read_text()

Using the recipe in the unittest docs doesn't patch this correctly? It must use builtins.open under the hood somewhere but where??

Answer 1

You can patch pathlib.Path.read_text directly. You only need to keep a reference to the original method, so you can decide whether to return a "fake" value, or execute the original method.

import pathlib
from unittest.mock import patch


path_to_mock = "foo/bar"
original_read_text = pathlib.Path.read_text

def read_text_side_effect(self: pathlib.Path, *args, **kwargs):
    # how you compare the paths depends on your use case.
    if self.absolute() == pathlib.Path(path_to_mock).absolute():
        return "mocked text"
    else:
        return original_read_text(self, *args, **kwargs)

with patch("pathlib.Path.read_text", autospec=True) as mock_read:
    mock_read.side_effect = read_text_side_effect

    print(pathlib.Path(path_to_mock).read_text())
    print(pathlib.Path("real_file").read_text())

Assuming "real_file" exists, you should see "mocked text" and the file's content in the output.