import re scan = re.compile(flags=re.VERBOSE, pattern=r""" (?P \d+\.?\d* ) | (?P \+ | - | \* | / ) | (?P $ ) | (?P \S ) # catch all (except whitespace) """).finditer def parse(source): parser = RPNParser() for each in scan(source): dispatch = getattr(parser, each.lastgroup) dispatch(each.group()) return parser.stack[0] class RPNParser(object): def __init__(self): self.stack = [] def number(self, token): self.stack.append(token) def operator(self, token): right = self.stack.pop() left = self.stack.pop() self.stack.append(Expression(left, token, right)) def eof(self, token): if len(self.stack) != 1: raise Exception("source kaputt!") def error(self, token): raise Exception("unknown token %s!" % token) class Expression(object): def __init__(self, *args): self.args = args def __str__(self): return "(%s %s %s)" % self.args if __name__ == "__main__": print parse("1.5 2 + 3.33 1.11 - *")