A way of implementing subtraction using addition only. Available operations: loop, assign, add by one. Restrictions: no negative numbers, integers only.

subtraction_by_addition.py

# A way of implementing subtraction using addition only.
# Available operations: loop, assign, add by one.
# Restrictions: no negative numbers, integers only.
#
# Aadit M Shah on StackOverflow says this method was devised by Stephen Cole Kleene, but I can't find any sources.
# https://stackoverflow.com/a/34079075


def subtract(minuend, subtrahend):
    for _ in range(subtrahend):
        minuend = _subtract_by_one(minuend)
    return minuend

def _subtract_by_one(n):
    difference = 0
    difference_plus_one = 0
    for _ in range(n):
        difference = difference_plus_one
        difference_plus_one += 1
    return difference

if __name__ == '__main__':
    from random import randint

    for _ in range(10):
        a = randint(0, 999)
        b = randint(0, a)
        print('{0:>3} - {1:>3} = {2:>3} got {3:>3}'.format(a, b, a - b, subtract(a, b)))