How do I patch the `pathlib.Path.exists()` method?

Question

I want to patch the exists() method of a pathlib.Path object for a unit test but I have problems getting this to work.

What I am trying to do is this:

from unittest.mock import patch
from pathlib import Path

def test_move_basic():
    p = Path('~/test.py')
    with patch.object(p, 'exists') as mock_exists:
        mock_exists.return_value = True

But it fails with:

AttributeError: 'PosixPath' object attribute 'exists' is read-only.

Any ideas?

Answer 1

You need to patch the class, not the instance. It is enough to patch the method on the Path class, as it defines the exists method for the whole of the pathlib library (PosixPath, WindowsPath, PurePosixPath and PureWindowsPath all inherit the implementation):

>>> from unittest.mock import patch
>>> from pathlib import Path
>>> p = Path('~/test.py')
>>> with patch.object(Path, 'exists') as mock_exists:
...     mock_exists.return_value = True
...     p.exists()
...
True
>>> with patch.object(Path, 'exists') as mock_exists:
...     mock_exists.return_value = False
...     p.exists()
...
False
>>> with patch.object(Path, 'exists') as mock_exists:
...     mock_exists.return_value = 'Anything you like, actually'
...     p.exists()
...
'Anything you like, actually'

The pathlib classes use __slots__ to keep their memory footprint low, which has the side-effect of their instances not supporting arbitrary attribute assignment.