A brainfuck interpreter (Not completely done, please report bugs)

bf.py

#Python Brainfuck interpreter, made by RK11
#Version 0.2
#Changelog:
#0.1> First version
#0.2> Better output, and debug mode

import sys, time
class bf:
	def __init__(self, input_bf, debug = False):
		self.input = input_bf
		self.bytes = [0]*1000000
		self.pointer = 0
		self.loops = []
		self.output = ""
		self.pos = 0
		self.find_loops()
		self.debug = debug
	def find_loops(self): #Find all loops, and the posistions of the matching brackets
		opening_brackets = []
		closing_brackets = []
		level = -1
		for pos in range(len(self.input)):
			if self.input[pos] == "[":
				opening_brackets.append(pos)
				closing_brackets.append(-1)
			if self.input[pos] == "]":
				a = len(closing_brackets)-1
				while a >= 0:
					if closing_brackets[a] == -1:
						closing_brackets[a] = pos
						break
					a -= 1
		for pos in range(len(opening_brackets)):
			self.loops.append([opening_brackets[pos],closing_brackets[pos]])
	def find_matching_bracket(self): #Return the matching bracket of a another bracket
		for twobrackets in self.loops:
			for pos in range(2):
				if self.pos == twobrackets[pos]:
					return twobrackets[1-pos]
			
	def ex(self): #Execute the program
		while self.pos < len(self.input):
			char = self.input[self.pos] #actual character
			if char == "+":
				self.bytes[self.pointer] += 1
				if self.bytes[self.pointer] > 255:
					self.bytes[self.pointer] = 0
			if char == "-":
				self.bytes[self.pointer] -= 1
				if self.bytes[self.pointer] < 0:
					self.bytes[self.pointer] = 255
			if char == "<":
				self.pointer -= 1
			if char == ">":
				self.pointer += 1
			if char == ".":
				self.output += chr(self.bytes[self.pointer] % 256)
			if char == "[":
				if self.bytes[self.pointer] == 0:
					self.pos = self.find_matching_bracket()
			if char == "]":
				if self.bytes[self.pointer]:
					self.pos = self.find_matching_bracket()
			if char == ",":
				self.bytes[self.pointer] = ord(raw_input("\n>"))
			#Update the output
			sys.stdout.write("\r{}          ".format(self.output))
			sys.stdout.flush()
			if self.debug:
				print ("{} {}".format(self.bytes[:10], char)) #change this value to print more or less bytes
				time.sleep(0.1)#You can increase the speed by decreasing this value
			self.pos += 1
		print "\n"
		self.output = 0
		
#usage:
#test = bf(">+++++++++++++[<+++++>-]>+++++[<+++++>-]<+[<.+>-]")
#test.ex()  #Should print the alphabet