switch 擬で記述すると

expr = "3 4 +"
expr = "3 4 + 2 *"

parser = Parser()
cal = Polish()
for e in expr.split(" "):
    e = parser.scan(e)
    e.switch({
        Value: cal.value,
        Op   : cal.op,
        })
print cal.evaluate()

# --------------------------

class Polish:
    def __init__(self):
        self.stack = []

    def value(self, e):
        self.stack.append(e)

    def op(self, e):
        op2 = self.stack.pop()
        op1 = self.stack.pop()
        value = eval("%s %s %s"%(op1, e, op2))
        self.stack.append(value)

    def evaluate(self):
        return self.stack.pop()

# --------------------------
class Token:
    def __repr__(self):
        return "%s"%self.value

    def switch(self, dispatch):
        return dispatch[self.__class__](e)

class Value(Token):
    def __init__(self, value):
        self.value = int(value)
        
class Op(Token):
    def __init__(self, value):
        self.value = str(value)        
    
# --------------------------