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
