When using a mock and deleting a attribute reset_mock cannot be used anymore since it tries to call reset_mock on the _deleted sentinel value.Reproduction path:```from unittest.mock import MagicMockmock = MagicMock()mock.a = 'test'del mock.amock.reset_mock()```Gives:```Traceback (most recent call last):  File "<stdin>", line 1, in <module>  File "/usr/lib/python3.5/unittest/mock.py", line 544, in reset_mock    child.reset_mock(visited)AttributeError: '_SentinelObject' object has no attribute 'reset_mock'```Expected result:mock is reset without throwing an exception and the 'a' attribute is no longer in a deleted stateOnly checked 3.5 and current master if bug is present

Can confirm this behavior on CPython master as well. It seems that when an attribute is deleted then a deleted flag is set for the attribute athttps://github.com/python/cpython/blob/73820a60cc3c990abb351540ca27bf7689bce8ac/Lib/unittest/mock.py#L737 . But when reset_mock is called it doesn't check for the deleted flag athttps://github.com/python/cpython/blob/73820a60cc3c990abb351540ca27bf7689bce8ac/Lib/unittest/mock.py#L543➜  cpython git:(master) ./python.exe ../backups/bpo31177.pyTraceback (most recent call last):  File "../backups/bpo31177.py", line 5, in <module>    m.reset_mock()  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/unittest/mock.py", line 546, in reset_mock    child.reset_mock(visited)AttributeError: '_SentinelObject' object has no attribute 'reset_mock'A simple patch would be to skip the deleted as below but some of the code in mock module raise an AttributeError. I don't know the correct behavior here. But applying the below patch and running tests with `./python.exeLib/unittest/test/` doesn't cause any test failure.diff --git a/Lib/unittest/mock.py b/Lib/unittest/mock.pyindexdb1e642c00..700e2fb8b9 100644--- a/Lib/unittest/mock.py+++ b/Lib/unittest/mock.py@@ -541,7 +541,7 @@ class NonCallableMock(Base):             self._mock_side_effect = None          for child in self._mock_children.values():-            if isinstance(child, _SpecState):+            if isinstance(child, _SpecState) or child is _deleted:                 continue             child.reset_mock(visited) I will try to make a PR if it's ok.Thanks

Adding Michael for thoughts on the fix and desired behavior. Removing 3.5 since only security fixes are accepted and adding 3.8 which is also affected.Thanks

I suggest that after reset_mock(), deleted attributes should be available again. In other words, their deletion is reset.I'm opening a PR to this effect.I reported this issue to testing-cabal's mock repo in May 2016 (https://github.com/testing-cabal/mock/pull/361), but my original PR there just avoided an exception without reinstating the deleted attribute.

New changesetedeca92c84a3b08902ecdfe987cde00c7e617887 by Victor Stinner (Xtreak) in branch 'master':bpo-31177: Skip deleted attributes while calling reset_mock (GH-9302)https://github.com/python/cpython/commit/edeca92c84a3b08902ecdfe987cde00c7e617887

New changesetc0566e0ff6c2dd1a8b814ecd65649605c090527b by Miss Islington (bot) in branch '3.6':bpo-31177: Skip deleted attributes while calling reset_mock (GH-9302)https://github.com/python/cpython/commit/c0566e0ff6c2dd1a8b814ecd65649605c090527b

New changeset422c1658b7d34fdc73c5fc895b135862103d1983 by Miss Islington (bot) in branch '3.7':bpo-31177: Skip deleted attributes while calling reset_mock (GH-9302)https://github.com/python/cpython/commit/422c1658b7d34fdc73c5fc895b135862103d1983

I am closing this as fixed since all the PRs were merged. Feel free to reopen this if needed. Thanks @mariocj89 and @vstinner for the review.