How GAE emulator limits list of available Python modules?

I installed Python Mock module using PIP.

When I try to开发者_JAVA技巧 import mock running under 'dev_appserver', GAE says that it can't find module 'mock'.

import mock works perfectly in Python interpreter.

I understand that dev_appserver behaves absolutely correctly because I can't install modules with PIP on GAE servers.

My question is how technically dev_appserver filters list of modules that can be loaded?

The dev_appserver uses import hooks to prevent importing modules that shouldn't be available. The relevant code is here, but be warned - it's easily the most complicated bit of the dev_appserver!