TypeError: unbound method when trying to mock a classmethod

Question

此脚本失败：

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：

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)

Answer 1

Python 函数是 descriptors 和 Python 将它们绑定到它们所查找的实例，或者在 classmethod 的情况下绑定到 class.因为您没有在替换函数上使用 classmethod 装饰器，所以它被错误地绑定了（作为常规方法，所以没有传入 cls）。

只需手动将目标包装在 classmethod 装饰器中：

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

这里我手动应用了 @classmethod 装饰器，但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

TypeError: unbound method when trying to mock a classmethod

TypeError: unbound method when trying to mock a classmethod

python

unit-testing

mocking

class-method