fractran interpreter in python

fractran.py

import sys

if len(sys.argv) != 3:
	sys.stderr.write("usage: " + sys.argv[0] + " PROGRAM INPUT\n")
	sys.exit(1)

inp = int(sys.argv[2])
sys.stdout.write(str(inp) + " ")

symbols = []
currnum = ""
with open(sys.argv[1], "r") as fobj:
	while True:
		byte = fobj.read(1)
		if byte.isdigit():
			currnum += byte
		elif byte == "/" or byte.isspace() or byte == "":
			if currnum:
				symbols.append(int(currnum))
			currnum = ""
			if byte == "/":
				symbols.append("/")
			elif byte == "":
				break

program = []
num = None
div = False
for symbol in symbols:
	if isinstance(symbol, int):
		if div:
			if num == None:
				sys.stderr.write("unexpected /\n")
				sys.exit(1)
			program.append((num, symbol))
			num = None
			div = False
		else:
			if num != None:
				program.append((num, 1))
			num = symbol
	elif symbol == "/":
		div = True
if num != None:
	program.append((num, 1))

cont = True
while cont:
	cont = False
	for i in program:
		if isinstance(i, tuple):
			(numerator, denominator) = i
			nf = numerator * inp
			if nf % denominator == 0:
				inp = nf / denominator
				sys.stdout.write(str(inp) + " ")
				cont = True
				break