python - 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

[EDIT]

Maybe the most interesting part of this question is Why I cannot patch somefunction.__call__?

Because the function don't use __call__'s code but __call__ (a method-wrapper object) use function's code.

I don't find any well sourced documentation about that, but I can prove it (Python2.7):

>>> def f():
...     return "f"
... 
>>> def g():
...     return "g"
... 
>>> f
<function f at 0x7f1576381848>
>>> f.__call__
<method-wrapper '__call__' of function object at 0x7f1576381848>
>>> g
<function g at 0x7f15763817d0>
>>> g.__call__
<method-wrapper '__call__' of function object at 0x7f15763817d0>

Replace f's code by g's code:

>>> f.func_code = g.func_code
>>> f()
'g'
>>> f.__call__()
'g'

Of course f and f.__call__ references are not changed:

>>> f
<function f at 0x7f1576381848>
>>> f.__call__
<method-wrapper '__call__' of function object at 0x7f1576381848>

Recover original implementation and copy __call__ references instead:

>>> def f():
...     return "f"
... 
>>> f()
'f'
>>> f.__call__ = g.__call__
>>> f()
'f'
>>> f.__call__()
'g'

This don't have any effect on f function. Note: In Python 3 you should use __code__ instead of func_code.

I Hope that somebody can point me to the documentation that explain this behavior.

You have a way to work around that: in utils you can define

class Utcnow(object):
    def __call__(self):
        return datetime.datetime.utcnow()


utcnow = Utcnow()

And now your patch can work like a charm.

---

Follow the original answer that I consider even the best way to implement your tests.

I've my own gold rule: never patch protected methods. In this case the things are little bit smoother because protected method was introduced just for testing but I cannot see why.

The real problem here is that you cannot to patch datetime.datetime.utcnow directly (is C extension as you wrote in the comment above). What you can do is to patch datetime by wrap the standard behavior and override utcnow function:

>>> with mock.patch("datetime.datetime", mock.Mock(wraps=datetime.datetime, utcnow=mock.Mock(return_value=3))):
...  print(datetime.datetime.utcnow())
... 
3

Ok that is not really clear and neat but you can introduce your own function like

def mock_utcnow(return_value):
    return mock.Mock(wraps=datetime.datetime, 
                     utcnow=mock.Mock(return_value=return_value)):

and now

mock.patch("datetime.datetime", mock_utcnow(***))

do exactly what you need without any other layer and for every kind of import.

Another solution can be import datetime in utils and to patch ***.utils.datetime; that can give you some freedom to change datetime reference implementation without change your tests (in this case take care to change mock_utcnow() wraps argument too).