Comment 1 for bug 1511474

I saw this the other day with charm-helpers. An old version of the mock library has been pinned in the Python virtual env, but if the newer version is installed in the system it takes precedence. Running the tests after doing 'apt get remove python-mock python3-mock' should tell you if you are seeing the same problem, or running the tests in a fresh lxc container where it was not installed in the first place.