f1(), f2() lambda functions addition

[micseydel]

Can you provide full code which reproduces the problem? Currently, you've provided the function definition but not the call. Since Python is dynamically typed, we can't know anything about f1 and f2 other than you're trying to call them and use the minus operator with their return values.

It might help to provide a full homework description, if you don't understand what I'm saying.