Return the Hamming distance between equal-length sequences

def hamming_distance(s1, s2):
    """Return the Hamming distance between equal-length sequences"""
#     print(s1,s2)
    if len(s1) != len(s2):
        raise ValueError("Undefined for sequences of unequal length")
    return sum(el1 != el2 for el1, el2 in zip(s1.upper(), s2.upper()))