stack datastructs implementation

infixtopostfix.py

class Stack:
	def __init__(self):
		self.items = []
	def isEmpty(self):
		return self.items == []
	def push(self, item):
		self.items.append(item)
	def pop(self):
		return self.items.pop()
	def peek(self):
		return self.items[len(self.items)-1]
	def size(self):
		return len(self.items)
import string*
def inftopost(infiexpr):
    prec={}
    prec["*"]={3}
    prec["/"]={3}
    prec["+"]={2}
    prec["-"]={2}
    prec["("]={1}
    k=Stack()
    plist=[]
    tok=infiexpr.split()
    for t in tok:
       if t in string.uppercase():
          plist.append(t)
       if t == "(":
           k.pop(t)
       if == in ")":
           tok = opStack.pop()
	   while tok != '(':
	       plist.append(tok)
		tok = k.pop()
        else:
	     while(not k.isEmpty()) and (prec[k.peek()]>=prec[t]):
		plist.append(k.pop())
		k.push(t)
	while not k.isEmpty():
		plist.append(k.pop())
	return string.join(pl)