This is a bit annoying:

>>> def f(a, b): pass
... 
>>> mock = Mock(spec=f)
>>> mock(1, 2)
<Mock name='mock()' id='140654219634288'>
>>> mock.assert_called_with(1, 2)
>>> mock.assert_called_with(a=1, b=2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/antoine/cpython/default/Lib/unittest/mock.py", line 726, in assert_called_with
    raise AssertionError(msg)
AssertionError: Expected call: mock(b=2, a=1)
Actual call: mock(1, 2)

This means your test assertions will depend unduly on some code style details (whether some function is called using positional or keyword arguments).
Note: if this is fixed, it should be made to work with method calls too.

(note: also fails if I use `mock = Mock(wraps=f)` instead of `mock = Mock(spec=f)`)

Proof-of-concept patch. mock is ugly!

Here is a new patch, making all assert_*_call methods work as well as autospeccing, and adding tests. Not sure I'm forgetting something else.

Wow, impressive work Antoine - thanks. I am a little concerned that this relies on Python 3 only features of inspect, and *in fact* relies on bug fixes in Python 3.4 to work. That means it would be hard / impossible for the backport "mock" to have the new feature. That may be solveable (there is a backport of function signatures which mock could use for example).

> I am a little concerned that this relies on Python 3 only features of
> inspect, and *in fact* relies on bug fixes in Python 3.4 to work.

The bug fix has landed in 3.3 as well ;-)
(see 49fd1c8aeca5)
I guess a backport of inspect.signature() would allow it to work on previous Pythons, too.

Ah, well if the bugfix exists everywhere that function signatures exist then it shouldn't be a problem. (I think there is already a backport of inspect.signature() by Nick Coghlan.)

New changeset b888c9043566 by Antoine Pitrou in branch 'default':
Issue #17015: When it has a spec, a Mock object now inspects its signature when matching calls, so that arguments can be matched positionally or by name.
http://hg.python.org/cpython/rev/b888c9043566

Committed!