doc: PropertyMock refuses to raise AttributeErrror as a side effect

[voidspace]

BPO	21254
Nosy	@rbtcollins, @voidspace, @ericsnowcurrently, @kushaldas, @s-t-e-v-e-n-k

^{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 = 'https://github.com/voidspace'
closed_at = None
created_at = <Date 2014-04-16.16:15:17.468>
labels = ['easy', 'type-bug', '3.8', '3.9', '3.10', 'library', 'docs']
title = 'doc: PropertyMock refuses to raise AttributeErrror as a side effect'
updated_at = <Date 2021-03-28.22:54:36.893>
user = 'https://github.com/voidspace'

bugs.python.org fields:

activity = <Date 2021-03-28.22:54:36.893>
actor = 'iritkatriel'
assignee = 'michael.foord'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Library (Lib)']
creation = <Date 2014-04-16.16:15:17.468>
creator = 'michael.foord'
dependencies = []
files = []
hgrepos = []
issue_num = 21254
keywords = ['easy']
message_count = 5.0
messages = ['216487', '216503', '275934', '275937', '275938']
nosy_count = 5.0
nosy_names = ['rbcollins', 'michael.foord', 'eric.snow', 'kushal.das', 'stevenk']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue21254'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

Linked PRs

• gh-65453: Docs - clarify AttributeError behaviour on PropertyMock #121666
• [3.13] gh-65453: Docs - clarify AttributeError behaviour on PropertyMock (GH-121666) #121968
• [3.12] gh-65453: Docs - clarify AttributeError behaviour on PropertyMock (GH-121666) #121969

[voidspace]

What steps will reproduce the problem?

>> import mock
>> a_mock = mock.MagicMock()
>> no_attribute = mock.PropertyMock(side_effect=AttributeError)
>> type(a_mock).property = no_attribute

What is the expected output? What do you see instead?

I would expect the above to raise an AttributeError. Instead it returns a MagicMock instance.

>>> a_mock.property
<MagicMock name='mock.property' id='140165240345424'>

I would expect it to have the same effect as calling a PropertyMock with any other exception as a side effect:

>>> mock_value_error = mock.PropertyMock(side_effect=ValueError)
>>> type(a_mock).other_property = mock_value_error
>>> a_mock.other_property
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ahammel/bin/python/mock-1.0.1-py2.6.egg/mock.py", line 2365, in __get__
    return self()
  File "/home/ahammel/bin/python/mock-1.0.1-py2.6.egg/mock.py", line 955, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/home/ahammel/bin/python/mock-1.0.1-py2.6.egg/mock.py", line 1010, in _mock_call
    raise effect
ValueError

What version of the product are you using? On what operating system?

Using version mock-1.0.1-py2.6 on CentOS 6.4

Please provide any additional information below.

PropertyMock objects apparently won't raise sublcasses of AttributeError either:

>>> class MockAttributeError(AttributeError): pass
... 
>>> no_attr = mock.PropertyMock(side_effect=MockAttributeError)
>>> type(a_mock).property = no_attr
>>> a_mock.property
<MagicMock name='mock.property' id='140165240345424'>

Works fine for subclasses of other Exceptions:

>>> class MockKeyError(KeyError): pass
... 
>>> no_key = mock.PropertyMock(side_effect=MockKeyError)
>>> type(a_mock).property = no_key
>>> a_mock.property
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ahammel/bin/python/mock-1.0.1-py2.6.egg/mock.py", line 2365, in __get__
    return self()
  File "/home/ahammel/bin/python/mock-1.0.1-py2.6.egg/mock.py", line 955, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/home/ahammel/bin/python/mock-1.0.1-py2.6.egg/mock.py", line 1010, in _mock_call
    raise effect
__main__.MockKeyError

[ericsnowcurrently]

Perhaps related to bpo-1615?

[rbtcollins]

We're poking at this at the kiwipycon sprints

[s-t-e-v-e-n-k]

This is in fact, working entirely as expected.

If we define a property that raises an AttributeError, then __getattr__() will be called, since the descriptor protocol says that an attribute error is a missing descriptor.

>>> class Foo(object):
...   def __getattr__(self, attr):
...     return 42
...   @property
...   def bacon(self):
...     print(1)
...     return int.lala
... 
>>> Foo().bacon
1
42

If we then follow a similar pattern using mock:

>>> from unittest import mock
>>> a_mock = mock.MagicMock()
>>> def foo():
...   print(1)
...   raise AttributeError()
... 
>>> no_attribute = mock.PropertyMock(side_effect=foo)
>>> type(a_mock).property = no_attribute
>>> a_mock.property
1
<MagicMock name='mock.property' id='139971099507232'>

You can see that the method is called, since we print one, but then what is going on is that MagicMock.__getattr__ is called, which has the behavior to return a new mock, like so:

>>> a_mock.b
<MagicMock name='mock.b' id='139971099646776'>

[rbtcollins]

I could imagine doing some complex things to let you get at the AttributeError in this case but *not* when a different descriptor was added to type(a_mock), but I think it would confusing overall. This might be worth a doc tweak to cover it?