Function to lock a file using memcache, Version 1
The code only acquires a lock for the source of the move, and not for the destination. This means that there's a race between two users trying to move two different files to the same destination.
There's a race between:

memcache.delete(filename)


and:

user = memcache.get(filename)

It's not clear how the lock is released. It doesn't happen in process.
The test else: if fileExists: seems backwards.