stack_calc.py

class Stack_calc(object):
    '''Stack calulator'''
    def __init__(self):
        self.item = []
        self.calculate =
         {'+' : lambda: self.item.append(self.item.pop() + self.item.pop()),
          '-' : lambda: self.item.append(self.item.pop() - self.item.pop()),
          '*' : lambda: self.item.append(self.item.pop() * self.item.pop()),
          '/' : lambda: self.item.append(self.item.pop() / self.item.pop())}

    def parse_stack(self, source):
        self.stack = source.split()
        for num in self.stack:
            try:
                self.item.append(float(num))
            except ValueError:
                for element in self.stack:
                    if element in ['+','-','*','/'] and len(self.item) > 1:
                        self.calculate[element]()
                    else:
                        try:
                            d = {'+': self.item[0] + self.item[0],'-': self.item[0] - self.item[0],
                                 '*': self.item[0] * self.item[0],'/': self.item[0] / self.item[0]}
                            if element in d:
                                self.item[0] = d[element]
                        except (IndexError,ZeroDivisionError):
                            pass

    def __str__(self):
        return str(self.item)

def main():
    '''Enter numbers,will be added to a stack
    use operators +-*/ to calculate 2 last elements
    To exit type Q
    '''
    obj = Stack_calc()
    while True:
        into_stack = raw_input(':-> ')
        if into_stack == 'Q':
            break
        obj.parse_stack("%s" % into_stack)
        print obj

if __name__ == '__main__':
    main()