This bug was introduced with Issue25597


Here's some code that demonstrates the error:

    import sys
    from unittest.mock import patch

    with patch.object(sys, 'stdout', wraps=sys.stdout) as mockstdout:
        bool(sys.stdout)

This works fine in 3.8 and earlier, but fails in 3.9

It seems the goal was to be able to access dunder methods for wrapped objects. Before this change __bool__ wasn't actually being checked, but was forced to True, which works for basic existence tests. The new code method._mock_wraps = getattr(mock._mock_wraps, name) has no fallthrough in case the attribute isn't there such as the case with __bool__ on sys.stdout.