unittest.mock.Mock.parent is broken or undocumented

[torfsen]

BPO	39222
Nosy	@cjw296, @voidspace, @lisroach, @mariocj89, @tirkarthi, @torfsen

^{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 = None
created_at = <Date 2020-01-05.14:58:31.979>
labels = ['type-bug', '3.8', '3.9', '3.7', 'library', 'docs']
title = 'unittest.mock.Mock.parent is broken or undocumented'
updated_at = <Date 2020-01-15.12:00:36.437>
user = 'https://github.com/torfsen'

bugs.python.org fields:

activity = <Date 2020-01-15.12:00:36.437>
actor = 'mariocj89'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Library (Lib)']
creation = <Date 2020-01-05.14:58:31.979>
creator = 'florian.brucker'
dependencies = []
files = []
hgrepos = []
issue_num = 39222
keywords = []
message_count = 7.0
messages = ['359348', '359578', '359724', '359725', '359726', '359890', '360044']
nosy_count = 7.0
nosy_names = ['cjw296', 'michael.foord', 'docs@python', 'lisroach', 'mariocj89', 'xtreak', 'florian.brucker']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39222'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

Linked PRs

• gh-83403: Test parent param in Mock.__init__ #103630

[torfsen]

The "parent" attribute of unittest.mock.Mock is either broken or undocumented.

For example, on Python 3.7.4:

>>> from unittest.mock import Mock
>>> m = Mock(x=1, parent=2)
>>> m.x
1
>>> m.parent
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.7/unittest/mock.py", line 659, in __repr__
    name = self._extract_mock_name()
  File "/usr/local/lib/python3.7/unittest/mock.py", line 638, in _extract_mock_name
    _name_list.append(_parent._mock_new_name + dot)
AttributeError: 'int' object has no attribute '_mock_new_name'
>>> parent = Mock()
>>> child = Mock(parent=parent)
>>> child.parent is parent
False

I stumbled upon this while trying to mock an object that has a "parent" attribute.

From the documentation I understand that mocks have built-in parents. However, the documentation never mentions the "parent" attribute specifically, so I always assumed that the built-in parent-child relationship was handled using private or name-mangled attributes. And since the "parent" attribute is not mentioned in the docs, I assumed I could set it by passing an additional kwarg to Mock.

I would have expected one of the following, in order of personal preference:

a) That a private or name-mangled attribute is used for the built-in parent-child relationship, so that I can mock objects which themselves have a "parent" attribute

b) That the special meaning of the "parent" attribute is documented, and that trying to set it directly (via the constructor or via attribute assignment, and without going through attach_mock) triggers a warning.

[tirkarthi]

It's a similar situation to name argument conflict [0] except that it's parent here. You can use configure_mock or attribute setting to do the same thing like below. parent is discussed in the docs to indicate the attributes being children like "a" being the child of parent "m" as you have mentioned. Adding others for thoughts.

>>> from unittest.mock import Mock
>>> m = Mock()
>>> m.configure_mock(parent=2)
>>> m.parent
2
>>> m.a()
<Mock name='mock.a()' id='4367727104'>

>>> m = Mock()
>>> m.parent = 2
>>> m.parent
2

[0] https://docs.python.org/3/library/unittest.mock.html#mock-names-and-the-name-attribute