foobar google

expr.py

import fileinput
import sys

EOF = -1
TERMINAL = 0x01
BINARY_TIMES = 0x02
BINARY_PLUS = 0x03

class Node():
	def __init__(self, tp, left, right):
		self.tp = tp
		self.left = left
		self.right = right

class Parser():
	
	def __init__(self, txt):
		self.txt = txt
		self.pos = 0
		self.status = EOF
		self.len = len(txt)
		self.next()
	
	def match(self, c):
		if self.look == c:
			self.next()
			return 1
		else:
			self.expected(c)

	def expected(self, msg):
			raise TypeError("unexpected token '{0}', expecting '{1}' at position {2}".format(self.look, msg, self.pos))
			sys.stdout.flush()
	
	def get_num(self):
		if not (self.look >= '0' and self.look <= '9'):
			self.expected('/[0-9]+/')
		else:
			retval = self.look
			self.next()
			return retval

	def next(self):
		if self.pos >= self.len:
			self.look = EOF
		else:
			self.look = self.txt[self.pos]
			self.pos += 1

	def term(self):
		left = self.factor()
		stack = []
		stack.append(left)
		while self.look == '*':
			self.match('*')
			right = self.factor()
			stack.append(Node(BINARY_TIMES, stack.pop(), right))
		
		return stack.pop()

	def factor(self):
		return Node(TERMINAL, self.get_num(), None)

	def expr(self):
		left = self.term()
		stack = []
		stack.append(left)
		while self.look == '+':
			self.match('+')
			right = self.term()
			stack.append(Node(BINARY_PLUS, stack.pop(), right))

		return stack.pop()

class Traversor():
	
	def __init__(self, tree):
		self.tree = tree

	def postorder(self, tree):
		return self.process(tree)	

	def process(self, tree):
		if tree.tp is TERMINAL:
			return int(tree.left, 10)
		if tree.tp is BINARY_TIMES:
			return self.process(tree.left) * self.process(tree.right)
		if tree.tp is BINARY_PLUS:
			return self.process(tree.left) + self.process(tree.right)

def main():
		while True:
			try:
				data = raw_input("> ")
				parser = Parser(data)
				traversor = Traversor(parser.expr())
				val = traversor.postorder(traversor.tree)
				sys.stdout.write(str(val))
				sys.stdout.write('\n')
				sys.stdout.flush()
			except TypeError as err:
				print err
			except EOFError as err:
				sys.stdout.write('\n')
				sys.stdout.write("Bye")
				sys.stdout.write('\n')
				sys.stdout.flush()
				sys.exit(0)

if __name__ == "__main__":
	main()