Brainf*ck like stack machine interpreter

bfs.py

#!/usr/bin/python

from __future__ import print_function
import sys

if len(sys.argv[1]) > 1:
    with open(sys.argv[1]) as src:
        program = src.read()
else:
    print("No input file.")
    sys.exit(0)

inst_ptr = 0
stck_ptr = 0
stck = [0]
inst = program[0]

while len(program) > inst_ptr:
    inst = program[inst_ptr]

    if inst == '>':
        stck.append(0)
    elif inst == '<':
        del stck[-1]
    elif inst == '+':
        stck[-2] += stck[-1]
        del stck[-1]
    elif inst == '-':
        stck[-2] -= stck[-1]
        del stck[-1]
    elif inst == '.':
        print(chr(stck[-1]), end="")
        del stck[-1]
    elif inst == ',':
        stck.append(0)
        stck[-1] = int(raw_input("-->"))
    elif inst == '[':
        if stck[-1] == 0:
            while program[inst_ptr] != ']':
                inst_ptr += 1
    elif inst == ']':
        if stck[-1] != 0:
            while program[inst_ptr] != '[':
                inst_ptr -= 1

    inst_ptr += 1