Patch __call__ of a function

Question

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() 

Then in my tests I do something like:

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

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.

This does not work for me:

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

How to do this elegantly?

Answer 1

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.

For example:

>>> 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 

Assigning anything to a.__call__ is pointless.

However:

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

TLDR;

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

Links

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?".

This behaviour is documented in Python documentation on Special method lookup.