Use dict primitive operations to avoid usage of locking

As far as I've understood Python, the following should not require any locks http://docs.python.org/2/glossary.html#term-global-interpreter-lock:
"The mechanism used by the CPython interpreter to assure that only one thread executes Python bytecode at a time. This simplifies the CPython implementation by making the object model (including critical built-in types such as dict) implicitly safe against concurrent access." All other implementations I know of that have no  GIL use explicit locking to imitate this behaviour.