what are some good ways of calling user defined function from a python library?

Let's say I am writing a library module a.py like this

def call_func(f):
    """ invoke func by its func str """
    return eval(f)()

and the user define function is in module b.py,

from a import call_func  
def b():
    return "b"

call_func("b")

This will be an error since in module a, function b is not exposed. Since function b is in user space, from library a point of view, user defined function b is not known.

It might be possible to use a register function in a.py, but I don't know how to do it. Or maybe there are alternative ways?

Thanks!