Brainfuck interpreter

bf.py

from collections import defaultdict
import sys

PTR = 0


class Memory(defaultdict):
    def __init__(self):
        super(Memory, self).__init__(lambda: 0)

MEM = Memory()


def evaltoken(token):
    global PTR
    global MEM

    for char in token:
        if char == '>':
            PTR += 1
        elif char == '<':
            PTR -= 1
        elif char == '+':
            MEM[PTR] += 1
        elif char == '-':
            MEM[PTR] -= 1
        elif char == '.':
            sys.stdout.write(chr(MEM[PTR]))
        elif char == ',':
            MEM[PTR] = ord(sys.stdin.read(1))


def evalbf(bf, loop=False):
    global PTR
    global MEM

    if bf:
        token = bf.pop(0)
    else:
        return

    if token == '[':
        while token != ']' and bf:
            evalbf(bf, loop=True)
        if bf:
            bf.pop(0)
        return
    else:
        evaltoken(token)
        while loop and MEM[PTR] != 0:
            evaltoken(token)

    if bf:
        evalbf(bf)


def main():
    global PTR
    global MEM

    while True:
        print
        bf = raw_input('in: ')

        if bf == 'dump':
            print PTR, MEM
            continue

        if bf == 'reset':
            PTR = 0
            MEM.clear()
            continue

        bf = bf.replace('[', ' [ ').replace(']', ' ] ').split()
        evalbf(bf)


if __name__ == '__main__':
    main()