divisible_by_7.py

def divisible_by_7(number):
    number = abs(number)
    if(number == 7 or number == 0):
        return True
    elif(number < 10):
        return False
    else:
        last_digit = number % 10
        other_digits = number // 10
        return divisible_by_7(other_digits-2*last_digit)

# test
for i in range(1000):
    if(not (i%7==0) == divisible_by_7(i)):
        print("ERROR", i, "is", "divisible by 7" if divisible_by_7(i) else "not divisible by 7")

print("done")