As others pointed out, abs is builtin so it isn't imported from the math module.

I wanted to comment on your type checking. Another way that is the most "pythonic" is to use a try: except: block to check the type:

def distance_from_zero(num):
    try:
        return abs(num)
    except ValueError:
        return "Not an numeric type!"

This takes care of the issue that F.J. pointed out, that long and complex won't be considered. This example uses "duck typing" (if it walks like a duck and quacks like a duck, it must be a duck). If abs works, your function succeeds. If you supply something abs doesn't know how to handle a ValueError will be raised an it will return your error message.

From what I see, the result of your code would always be pi or 0. It will be pi if one of the mu1 or mu2 is negative and when both are negative or positive it will be zero. If I remember vectors properly : Given two vectors P and Q, with say P = (x, y) and Q = (a, b) Then abs(P) = sqrt(x^2 + y^2) and P. Q = xa+yb. So that cos@ = P. Q/(abs(P) *abs(Q)). If am not clear you can give an example of what you intend to do