[unittest.mock] `spec_set` should be applied after setting attributes via kwargs

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
42/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
testing

Research direction

Start at unittest.mock.create_autospec() and follow how its keyword arguments reach configure_mock() and spec_set enforcement. Reproduce the example from the issue, then verify that kwargs such as banana are applied before spec_set rejects later unknown attributes.

Written by the indexing model from the issue text.

Description

stdlib type-feature 
>>> a=create_autospec("foo", spec_set=True, banana=1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.8/unittest/mock.py", line 2607, in create_autospec
    mock = Klass(parent=_parent, _new_parent=_parent, _new_name=_new_name,
  File "/usr/lib/python3.8/unittest/mock.py", line 2006, in __init__
    _safe_super(MagicMixin, self).__init__(*args, **kw)
  File "/usr/lib/python3.8/unittest/mock.py", line 460, in __init__
    self.configure_mock(**kwargs)
  File "/usr/lib/python3.8/unittest/mock.py", line 629, in configure_mock
    setattr(obj, final, val)
  File "/usr/lib/python3.8/unittest/mock.py", line 744, in __setattr__
    raise AttributeError("Mock object has no attribute '%s'" % name)
AttributeError: Mock object has no attribute 'banana'

One reason to pass extra kwargs to create_autospec() like this is to set missing attributes (e.g. because you know that they get set dynamically later on). So, it would be nice to have those attributes set on the mock before spec_set comes into effect.

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