It has nothing to do with __all__, and everything to do with the way namespaces work in Python.  'from urllib.request import urllib' creates a name 'urllib' in the global namespace of your module pointing to the urlopen function (*before* you do your patch), and patch has no effect on the global namespace of your module, only on the global namespace of urllib.request.  By contrast, urllib in your module's global namespace points to the urllib module, so urllib.request points to the urllib.request's global namespace, which patch has altered to point to your mock by the time you print its value.