TypeError:尝试模拟classmethod时的未绑定方法

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()

例外:

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函数是描述符,Python将它们绑定到它们被查看的实例,或者在类的情况下classmethod.因为你没有classmethod在替换函数上使用装饰器,所以它被错误地绑定(作为常规方法,所以没有cls传入).

只需classmethod手动将目标包装在装饰器中:

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

这里我@classmethod手动应用了装饰器,但您也可以直接在目标函数上使用它作为装饰器:

@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