python 不使用 if else

#!/usr/bin/python

import sys

class Cacl:
    def __init__(self, x, y, op):
       self.x = x
       self.y = y
       self.op = op

    # Calculation method
    def add(self):
        return self.x + self.y
    def sub(self):
        return self.x - self.y
    def mul(self):
        return self.x * self.y
    def div(self):
         return self.x / self.y

    # Deal ERROR
    def op_error(self):
        print 'Calculation types are not supported: ', self.op
        sys.exit()

    # External interface
    def compute(self):
        try:
            return {
                '+' : self.add(),
                '-' : self.sub(),
                '*' : self.mul(),
                '/' : self.div()
             }[self.op]
        except:
            self.op_error()

def main():
    # get input str
    # debug: # in_str = '1 - 2'

    in_str = str(raw_input("input like a + b then Enter: "))
    try:
        x = float(in_str.split(' ')[0])
        op = in_str.split(' ')[1]
        y = float(in_str.split(' ')[2])
    except:
        print 'input error'
        sys.exit()

    print in_str + ' = ' + str(Cacl(x, y, op).compute())

if __name__ == '__main__':
    main()