Brainfuck interpreter in Python

bfi.py

#!/usr/bin/env python3
# Usage: ./bfi.py source.bf
import sys

with open(sys.argv[1]) as f:
    program = f.read()
    cells = [0]*30000
    ptr = 0
    pc = 0
    label_stack = []
    while pc < len(program):
        i = program[pc]
        if i == '>':
            ptr += 1
        elif i == '<':
            ptr -= 1
        elif i == '+':
            cells[ptr] = (cells[ptr] + 1) % 256
        elif i == '-':
            cells[ptr] = (cells[ptr] - 1) % 256
        elif i == '.':
            sys.stdout.write(chr(cells[ptr]))
            sys.stdout.flush()
        elif i == ',':
            cells[ptr] = ord(sys.stdin.read(1))
        elif i == '[':
            if cells[ptr] == 0:
                c  = 0
                while True:
                    pc += 1
                    if program[pc] == '[':
                        c += 1
                    elif program[pc] == ']':
                        c -= 1
                        if c == -1:
                            break
            else:
                label_stack.append(pc)
        elif i == ']':
            pc = label_stack.pop() - 1
        pc += 1