python – TypeError：尝试模拟classmethod时的未绑定方法

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)

解决方法

Python函数是 descriptors,Python将这些函数绑定到它们被查找的实例,或者在classmethod的情况下,绑定到类.因为你没有在替换函数上使用classmethod装饰器,所以它被错误地绑定(作为常规方法,因此没有传入cls).

只需手动将目标包装在classmethod装饰器中：

with mock.patch.object(MyClass,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,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,classmethod(mocked_method)):
...     MyClass.my_method()
... 
I want this method to get called