python - TypeError : unbound method when trying to mock a classmethod

该脚本失败:

import mock

class MyClass(object):

    @classmethod
    def my_method(cls):
        print('my_method')

def mocked_method(cls):
    print('I want this method to get called')

with mock.patch.object(MyClass, 'my_method', mocked_method):
    MyClass.my_method()

异常(exception):

Traceback (most recent call last):
  File "/home/foo/tmp/test_mocking_classmethod.py", line 14, in <module>
    MyClass.my_method()
TypeError: unbound method mocked_method() must be called with MyClass instance as first argument (got nothing instead)

最佳答案

Python函数是descriptors，Python将它们绑定(bind)到要查找的实例，或者在classmethod的情况下绑定(bind)到该类。因为您没有在替换函数上使用classmethod装饰器，所以它的绑定(bind)错误(作为常规方法，因此没有传递cls)。

只需将目标手动包裹在classmethod装饰器中:

with mock.patch.object(MyClass, 'my_method', classmethod(mocked_method)):
    MyClass.my_method()

在这里，我手动应用了@classmethod装饰器，但是您也可以直接在目标函数上按预期使用它作为装饰器:

@classmethod
def mocked_method(cls):
    print('I want this method to get called')

with mock.patch.object(MyClass, 'my_method', mocked_method):
    MyClass.my_method()

演示:

>>> import mock
>>> class MyClass(object):
...     @classmethod
...     def my_method(cls):
...         print('my_method')
...
>>> def mocked_method(cls):
...     print('I want this method to get called')
...
>>> with mock.patch.object(MyClass, 'my_method', classmethod(mocked_method)):
...     MyClass.my_method()
...
I want this method to get called