Minipig Interpreter

# An interpreter for MP, a.k.a. Minimalistic Pig
# An MP source code file is .mp0

import copy

class Instr:
    up = ["u","^"]
    down = ["d","v"]
    swap = ["s","/"] # swap the top two elements
    switch = ["S",";"] # swap working stacks
    one = ["l","1"]
    minus = ["m","-"]
    getnum = ["i","["]
    getchar = ["I","{"]
    printnum = ["o","]"]
    printchar = ["O","}"]
    flag = ["f","*"]
    goto = ["g",">"]
    reverse = ["r","%"]

with open("program.mp0") as f:
    prog = f.read()

alt = []
stack = []

def pop():
    global stack, ip

    if len(stack) == 0:
        print("There was an error on char {0}!".format(ip))
    top = stack[-1]
    del stack[-1]
    return top

def push(x):
    global stack
    stack.append(x)

ip = 0

length = len(prog)

register = 0

while ip < length:

    c = prog[ip]

    if c in Instr.up:
        register = pop()

    elif c in Instr.down:
        push(register)

    elif c in Instr.swap:
        a = pop()
        b = pop()
        push(a)
        push(b)

    elif c in Instr.switch:
        temp = copy.copy(stack)
        stack = copy.copy(alt)
        alt = copy.copy(temp)
        del temp

    elif c in Instr.one:
        push(1)

    elif c in Instr.minus:
        a = pop()
        b = pop()
        push(b-a)

    elif c in Instr.getnum:
        push(int(input()))

    elif c in Instr.getchar:
        raise RuntimeError("error on {0} - { is not implemented".format(ip))

    elif c in Instr.printnum:
        print(pop(), end="")

    elif c in Instr.printchar:
        print(chr(pop()), end="")

    elif c in Instr.reverse:
        stack.reverse()

    elif c in Instr.goto:
        j = pop()
        if j < 0:
            for _ in range(-j):
                ip -= 1
                while prog[ip] not in Instr.flag:
                    ip -= 1
        elif j > 0:
            for _ in range(j):
                ip += 1
                while prog[ip] not in Instr.flag:
                    ip += 1

    ip += 1