The following fails on python 3.4.2, 3.4.3 and 3.5.0a2 (downloaded from python.org and compiled on Ubuntu 14.04). 
The same happens when using mock.patch with autospec=True.

>>> from unittest.mock import create_autospec
>>> def function():
...     pass
... 
>>> mock = create_autospec(function)
>>> mock.side_effect = ValueError('MyError')
>>> mock()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 3, in function
  File "/usr/local/lib/python3.5/unittest/mock.py", line 910, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/usr/local/lib/python3.5/unittest/mock.py", line 963, in _mock_call
    effect = self.side_effect
  File "/usr/local/lib/python3.5/unittest/mock.py", line 510, in __get_side_effect
    sf = _MockIter(sf)
  File "/usr/local/lib/python3.5/unittest/mock.py", line 351, in __init__
    self.obj = iter(obj)
TypeError: 'ValueError' object is not iterable

But, on Python 3.3.5, 3.4.0, or when the mock is created via Mock(), for  instance, the exception is raised as expected:

[...]
>>> mock()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 3, in function
  File "/usr/lib/python3.4/unittest/mock.py", line 885, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/usr/lib/python3.4/unittest/mock.py", line 941, in _mock_call
    raise effect
ValueError: MyError

The problem only affects autospecced functions.

Apparently, the problem lies here (all code excerpts from Lib/unittest/mock.py):

- When autospeccing functions, the Mock._mock_delegate field is populated at the end of _setup_func (and its the only place I found where the delegate is set):

197 def _setup_func(funcopy, mock):
[...]
237     mock._mock_delegate = funcopy

- Mock.side_effect is a property, and proxies the get/set to _mock_delegate when it exists, and on the way out does not detect the exception and tries to make an _IterMock out of it and everything explodes.

 504     def __get_side_effect(self):
 505         delegated = self._mock_delegate
 506         if delegated is None:
 507             return self._mock_side_effect
 508         sf = delegated.side_effect
 509         if sf is not None and not callable(sf) and not isinstance(sf, _MockIter):
 510             sf = _MockIter(sf)

I've attached a patch which adds a test for this use case, and a proposed fix. Hope it helps :)

Also reported in the mock project as https://github.com/testing-cabal/mock/issues/264

This looks fine to me, I'm going to apply it.

New changeset db825807ab04 by Robert Collins in branch 'default':
Issue #23661: unittest.mock side_effects can now be exceptions again.
https://hg.python.org/cpython/rev/db825807ab04

New changeset 7021d46c490e by Robert Collins in branch '3.5':
Issue #23661: unittest.mock side_effects can now be exceptions again.
https://hg.python.org/cpython/rev/7021d46c490e

New changeset 231bf0840f8f by Robert Collins in branch 'default':
Issue 23661: null-merge with 3.5.
https://hg.python.org/cpython/rev/231bf0840f8f

python 3.4.3 is also affected. Is it possible to fix this in branch 3.4?