impl/python.py

def _is_int(value: object) -> bool:
    return isinstance(value, int) and not isinstance(value, bool)


def meter_advance(opening_read: int, closing_read: int, digits: int) -> int:
    """Units a meter advanced between two reads. A closing read lower than the
    opening read means the register rolled over past all nines, once.
    """
    if not _is_int(digits) or digits < 1 or digits > 15:
        raise ValueError("digits must be a whole number from 1 to 15, received %s" % (digits,))
    span = 10**digits
    for read in (opening_read, closing_read):
        if not _is_int(read) or read < 0 or read >= span:
            raise ValueError(
                "reads must be whole numbers from 0 to %d for a %d-digit meter, received %s" % (span - 1, digits, read)
            )
    if closing_read >= opening_read:
        return closing_read - opening_read
    return closing_read + span - opening_read