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