python - 如何修补 `pathlib.Path.exists()` 方法？

Question

我想为单元测试修补对象的exists()方法，pathlib.Path但我无法让它工作。

我想做的是：

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。修补类上的方法就足够了Path，因为它定义了exists整个pathlib库的方法（PosixPath, WindowsPath，PurePosixPath并且PureWindowsPath都继承了实现）：

>>> 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__保持它们的内存占用较低，这具有它们的实例不支持任意属性分配的副作用。