一个面向对象的简单计算器(python)

#coding=utf8
import string

def count(numA, numB, operator):
    try:
        numberA = string.atof(numA)
        numberB = string.atof(numB)
    except:
        exit("input error,please check it number A :'%s' --  number B:'%s'" % (numA, numB))
    op = OperationFactory.createOperate(operator)

    op.setA(numberA)
    op.setB(numberB)
    return op.getResult()

class OperationFactory(object):
    '''
    工厂方法，生成对应操作类
    '''
    @staticmethod
    def createOperate(operate):
        operators = {
            '+':OperationAdd(),
            '-':OperationSub(),
            '*':OperationMul(),
            '/':OperationDiv(),
        }
        try:
            return operators[operate]
        except KeyError:
            exit("Temporarily does not support '%s' operator" % operate)

class Operation(object):
    '''
    操作父类，在需要时可以增加相应子类，如取模类OperationMod,复数类等，增加子类不影响其他子类的运算，
    达到封装的目的
    '''
    _numberA = 0
    _numberB = 0

    def NumberA(self):
        return self._numberA

    def NumberB(self):
        return self._numberB

    def getResult(self):
        return 0

    def setA(self, A):
        self._numberA = A

    def setB(self, B):
        self._numberB = B

class OperationAdd(Operation):
    def getResult(self):
        result = self.NumberA() + self.NumberB()
        return result

class OperationSub(Operation):
    def getResult(self):
        result = self.NumberA() - self.NumberB()
        return result

class OperationMul(Operation):
    def getResult(self):
        result = self.NumberA() * self.NumberB()
        return result

class OperationDiv(Operation):
    def getResult(self):
        result = self.NumberA() / self.NumberB()
        return result

if __name__ == '__main__':
    left = raw_input('number A: ')
    sign = raw_input('请输入 + - * / :')
    right = raw_input('number B: ')
    rs = count(left, right, sign)
    print rs