python something like assertFunctionNotCalled?

In python unit testing, is there a way to assert that a given block of code is not called?

I have a function I am testing that can follow either of two branches: a faster, more reliable branch if the user has logged in before, or a slower less reliable branch if this is their first visit. Basically, the first time a user logs in, the program has to request information about them from separate server, which can be slow (and, occasionally, down). Once it has the information, however, it caches it locally and should just use the local cache for the data on subsequent logins.

The end result of the function is the same either way, so there is no way for me to know which code branch it took to get the information by simply looking at the output of the function. So ideally I'd like to be able to do effectively an assertFunctionNotCalled to test that it is pulling from the local cache on the second (and subsequent) login attempt. How should I best test this?