Python function which verifies UPCs check digit

upc-check.py

def calc_check_digit(value):
    """calculate UPCA and UPCE check digit"""
    check_digit = 0
    odd_pos = True

    for char in str(value)[::-1]:
        if odd_pos:
            check_digit += int(char) * 3
        else:
            check_digit += int(char)
        odd_pos = not odd_pos  # alternate
    check_digit = check_digit % 10
    check_digit = 10 - check_digit
    check_digit = check_digit % 10

    return check_digit


def is_valid(upc):
    return int(calc_check_digit(int(str(upc)[0:-1]))) == int(str(upc)[-1])