Thanks for the review Pamela, indeed self.vfs is not defined.

The patched test_unittest runs without failure because the list_dir() nested function is never called. It is also never called in the un-patched version. Actually the un-patched test_discover_start_dir_is_package_calls_package_load_tests runs without failure when changing '__init__.py' of vfs to 'foo' or whatever :(