Brainfuck interpreter in Python

bfi.py

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

with open(sys.argv[1]) as f:
    py_program = "import sys;cells = [0]*30000;ptr = 0\n"
    level = 0
    for i in f.read():
        py_program += " "*level
        if i == '>':
            py_program += "ptr += 1"
        elif i == '<':
            py_program += "ptr -= 1"
        elif i == '+' or i == '-':
            py_program += "cells[ptr] = (cells[ptr]" + i + "1) % 256"
        elif i == '.':
            py_program += "sys.stdout.write(chr(cells[ptr]));sys.stdout.flush()"
        elif i == ',':
            py_program += "cells[ptr] = ord(sys.stdin.read(1))"
        elif i == '[':
            py_program += "while cells[ptr]:"
            level += 1
        elif i == ']':
            py_program +="pass"
            level -=1
        py_program += "\n"
    exec(py_program)