In Python 3.8 and later creating a mock with a spec specifying an object containing a property that happens to raise an exception when accessed will fail, because _mock_add_spec calls getattr() on every attribute of the spec. This did not happen in Python 3.6/3.7.

This is likely a fairly unusual scenario (and in the particular case where I encountered this I could just use a class instead of an instance for the spec), but it was surprising.

For example:

# cat test.py
from unittest import mock

class Foo:
    @property
    def bar(self) -> str:
        raise Exception('xxx')

m = mock.MagicMock(spec=Foo())

# python3.11 test.py
Traceback (most recent call last):
  File "/root/test.py", line 8, in <module>
    m = mock.MagicMock(spec=Foo())
        ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/unittest/mock.py", line 2069, in __init__
    _safe_super(MagicMixin, self).__init__(*args, **kw)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/unittest/mock.py", line 1087, in __init__
    _safe_super(CallableMixin, self).__init__(
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/unittest/mock.py", line 442, in __init__
    self._mock_add_spec(spec, spec_set, _spec_as_instance, _eat_self)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/unittest/mock.py", line 497, in _mock_add_spec
    if iscoroutinefunction(getattr(spec, attr, None)):
                           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/test.py", line 6, in bar
    raise Exception('xxx')
    ^^^^^^^^^^^^^^^^^^^^^^
Exception: xxx