Question

from unittest.mock import patch


class A:
    def b(self):
        return 'HAHAHA A'
a = A()

with patch('__main__.A') as a_mock:
    a_mock.b.return_value = 'not working'
    print(a.b())

HAHAHA A
>>>

为什么不打印'not working'？那么a_mock是什么？ ______________

Answer 1

您将全班替换为您的补丁，而不是现有班级上的方法。 a = A()在替换类之前创建了A的实例，因此a.__class__仍引用实际的类，而不是模拟。

模拟只能一次替换一个引用，而不是引用的对象。在修补程序之前，名称A和属性a.__class__都是对类对象的引用。然后，您只修改了A引用，保留a.__class__。

换句话说，a.__class__未修补，只有A，而A().b() 将打印not working。

您必须在课程上修补方法，以便a.__class__仍引用A，a.b将解析为修补了A.b mock：

with patch('__main__.A.b') as b_mock:
    b_mock.return_value = 'working as long as you patch the right object'
    print(a.b())

演示：

>>> with patch('__main__.A.b') as b_mock:
...     b_mock.return_value = 'working as long as you patch the right object'
...     print(a.b())
...
working as long as you patch the right object

不幸的是，您不能使用a.__class__修补Mock引用; Python只允许您使用该属性的实际类。

>>> with patch('__main__.a.__class__') as a_class_mock:
...     a_class_mock.b.return_value = 'working as long as you patch the right object'
...     print(a.b())
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mjpieters/Development/Library/buildout.python/parts/opt/lib/python3.5/unittest/mock.py", line 1312, in __enter__
    setattr(self.target, self.attribute, new_attr)
TypeError: __class__ must be set to a class, not 'MagicMock' object

Answer 2

您可能希望修补您的实例而不是您的课程：

with patch('__main__.a') as a_mock:
    a_mock.b.return_value = 'works perfectly ;)'
    print(a.b())

重用Mock来创建属性模块unittest.patch

2 个答案: