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.