Setting a exception side_effect on a mock from create_autospec does not work

[IgnacioRossi]

BPO	23661
Nosy	@rbtcollins, @voidspace, @berkerpeksag
Files	autospec_exception.patch: Test and fix for autospeccing functions with exceptions

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2015-07-16.01:55:59.296>
created_at = <Date 2015-03-14.00:45:40.133>
labels = ['type-bug', 'library']
title = 'Setting a exception side_effect on a mock from create_autospec does not work'
updated_at = <Date 2015-11-18.11:13:20.690>
user = 'https://bugs.python.org/IgnacioRossi'

bugs.python.org fields:

activity = <Date 2015-11-18.11:13:20.690>
actor = 'and'
assignee = 'none'
closed = True
closed_date = <Date 2015-07-16.01:55:59.296>
closer = 'berker.peksag'
components = ['Library (Lib)']
creation = <Date 2015-03-14.00:45:40.133>
creator = 'Ignacio Rossi'
dependencies = []
files = ['38479']
hgrepos = []
issue_num = 23661
keywords = ['patch']
message_count = 7.0
messages = ['238064', '238071', '246707', '246708', '246709', '246716', '254842']
nosy_count = 6.0
nosy_names = ['rbcollins', 'michael.foord', 'python-dev', 'berker.peksag', 'and', 'Ignacio Rossi']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue23661'
versions = ['Python 3.4', 'Python 3.5', 'Python 3.6']

[IgnacioRossi]

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

[IgnacioRossi]

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 :)

[rbtcollins]

Also reported in the mock project as testing-cabal/mock#264

[rbtcollins]

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

[python-dev]

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

[python-dev]

New changeset 7021d46c490e by Robert Collins in branch '3.5':
Issue bpo-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':
bpo-23661: null-merge with 3.5.
https://hg.python.org/cpython/rev/231bf0840f8f

[and]

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