修补函数的__call__ [英] Patch __call__ of a function

问题描述

我需要在测试中修补当前日期时间.我正在使用此解决方案:

I need to patch current datetime in tests. I am using this solution:

def _utcnow():
    return datetime.datetime.utcnow()


def utcnow():
    """A proxy which can be patched in tests.
    """
    # another level of indirection, because some modules import utcnow
    return _utcnow()

然后在我的测试中，我做类似的事情:

Then in my tests I do something like:

    with mock.patch('***.utils._utcnow', return_value=***):
        ...

但是今天有了一个主意，我可以通过修补函数utcnow的__call__而不是增加一个_utcnow来简化实现.

But today an idea came to me, that I could make the implementation simpler by patching __call__ of function utcnow instead of having an additional _utcnow.

这对我不起作用:

    from ***.utils import utcnow
    with mock.patch.object(utcnow, '__call__', return_value=***):
        ...

如何优雅地做到这一点?

How to do this elegantly?

推荐答案

在修补函数的__call__时，您正在设置该实例的__call__属性. Python实际上调用了在类上定义的__call__方法.

When you patch __call__ of a function, you are setting the __call__ attribute of that instance. Python actually calls the __call__ method defined on the class.

例如:

>>> class A(object):
...     def __call__(self):
...         print 'a'
...
>>> a = A()
>>> a()
a
>>> def b(): print 'b'
...
>>> b()
b
>>> a.__call__ = b
>>> a()
a
>>> a.__call__ = b.__call__
>>> a()
a

将任何内容分配给a.__call__是没有意义的.

Assigning anything to a.__call__ is pointless.

但是:

>>> A.__call__ = b.__call__
>>> a()
b

TLDR;

a()不调用a.__call__.它会调用type(a).__call__(a).

TLDR;

a() does not call a.__call__. It calls type(a).__call__(a).

在回答为什么type(x).__enter__(x)而不是Python中的x.__enter__()标准contextlib?" .

There is a good explanation of why that happens in answer to "Why type(x).__enter__(x) instead of x.__enter__() in Python standard contextlib?".

此行为记录在特殊方法查找的Python文档中.

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