def get_first_mismatch_index(array1, array2):
    """
    Compares two arrays and returns the index of the
    first unequal item or None if both arrays are equal
    """
    if len(array1) != len(array2):
        raise Exception("arrays must be same size")
    for pos in range(len(array1)):
        if (array1[pos] != array2[pos]):
            return pos
    return None