Brainfuck interpreter written in Python 3

bfi.py

#!/usr/bin/env python3

import os, sys

if not sys.argv[1:]:
	sys.exit("Usage: bfi.py file [input]")

if not os.path.isfile(sys.argv[1]):
	sys.exit("File not found.")

# Interpret a bf program
with open(sys.argv[1]) as f:
	bf = list(filter(lambda x: x in "+-><.,[]", f.read()))

# Input processing
userinput = sys.argv[2] if sys.argv[2:] else ""

pos = 0             # Instruction
array = {0:0}       # Cells
current = 0         # Current cell, incremented with '>' and decremented with '<'

while pos < len(bf):

	# Current instruction
	inst = bf[pos]

	# Increment pointer
	if inst == "+":
		if current not in array:
			array[current] = 1
		else:
			array[current] += 1
		array[current] %= 256 # Wrap around the ASCII table

	# Decrement pointer
	elif inst == "-":
		if current not in array:
			array[current] = -1
		else:
			array[current] -= 1
		array[current] %= 256 # Wrap around the ASCII table

	# Next cell
	elif inst == ">":
		current += 1
		if current not in array:
			array[current] = 0

	# Previous cell
	elif inst == "<":
		current -= 1
		if current not in array:
			array[current] = 0

	# Output ASCII character of cell value
	elif inst == ".":
		sys.stdout.write(chr(array[current]))

	elif inst == "," and userinput:
		array[current] = ord(userinput[0])
		userinput = userinput[1:]

	# Start loop
	elif inst == "[":
		if array[current] == 0:
			loops = 1
			while loops:
				pos += 1
				if bf[pos] == "[":
					loops += 1
				elif bf[pos] == "]":
					loops -= 1

	# End loop
	elif inst == "]":
		loops = 1
		while loops:
			pos -= 1
			if bf[pos] == "[":
				loops -= 1
			elif bf[pos] == "]":
				loops += 1
		pos -= 1

	pos += 1