The error handling in mock.decorate_callable (3.5-3.7) and mock.decoration_helper (3.8-3.9) is incorrectly implemented. If the error handler is triggered in the loop, the `patching` variable is out of scope and raises an unhandled `UnboundLocalError` instead.

This happened as a result of a 3rd-party library that attempts to clear the `patchings` list of a decorated function. The below code shows a recreation of the incorrect error handling:

import functools
from unittest import mock


def is_valid():
    return True


def mock_is_valid():
    return False


def decorate(f):
    @functools.wraps(f)
    def decorate_wrapper(*args, **kwargs):
        # This happens in a 3rd-party library
        f.patchings = []
        return f(*args, **kwargs)

    return decorate_wrapper


@decorate
@mock.patch('test.is_valid', new=mock_is_valid)
def test_patch():
    raise Exception()

After further investigation, it seems this was fixed in https://github.com/python/cpython/commit/436c2b0d67da68465e709a96daac7340af3a5238

However, this fix was as part of an unrelated changeset and in a different function in 3.8+, and was never rolled back to 3.7 and below. PR opened to add the missing attribute instantiation to 3.7.

I think that the current code is not correct. __exit__ should not be called if __enter__ is failed. If some __enter__ implementation calls other __enter__s it should manually call corresponding __exit__s.

New changeset 4b222c9491d1700e9bdd98e6889b8d0ea1c7321e by Serhiy Storchaka in branch 'master':
bpo-40126: Fix reverting multiple patches in unittest.mock. (GH-19351)
https://github.com/python/cpython/commit/4b222c9491d1700e9bdd98e6889b8d0ea1c7321e

Issue still exists in 3.7 and below, as it was part of a different function before. Current PR doesn't resolve the original issue that was raised.

New changeset ee249d798ba08f065efbf4f450880a446c6ca49d by Serhiy Storchaka in branch '3.8':
[3.8] bpo-40126: Fix reverting multiple patches in unittest.mock. (GH-19351) (GH-19483)
https://github.com/python/cpython/commit/ee249d798ba08f065efbf4f450880a446c6ca49d

New changeset 4057e8f9b56789223a1e691d7601003aceb84ad1 by Serhiy Storchaka in branch '3.7':
[3.7] bpo-40126: Fix reverting multiple patches in unittest.mock. (GH-19351) (GH-19484)
https://github.com/python/cpython/commit/4057e8f9b56789223a1e691d7601003aceb84ad1

3.5 and 3.6 are in security fixes only mode, and this issue is not a security issue. After merging PR 19484 I cannot reproduce the original issue anymore.

I left this issue open for the case if I find a way to write tests for the merged changes.