如何修补pathlib.Path.exists（）方法？

Question

我想修补对象的exists()方法pathlib.Path以进行单元测试，但是我无法使它起作用。

我想做的是这样的：

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

但是它失败了：

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

有任何想法吗？

Answer 1

您需要修补类，而不是实例。这足以贴剂的制造方法的Path类，因为它规定了exists对整个法pathlib库（PosixPath，WindowsPath，PurePosixPath和PureWindowsPath所有继承的实现）：

>>> 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'

这些pathlib类用于__slots__保持较低的内存占用量，这具有其实例不支持任意属性分配的副作用。