问在python中使用defaultdict时出错。我是python新手，我得到了AttributeError：'str‘对象没有'append’属性

EN

#!/usr/bin/py

from collections import defaultdict
"i am getting str object has no attribute appened"
class car:

    def __init__(self,car_name):

    self.car_name = car_name


class model(car):

    def __init__(self,car_model,car_name):

    self.car_model=car_model

    car.__init__(self,car_name)

cars1=defaultdict(list)

if __name__ == "__main__":

    i=1

    while(i<=2):

        car_name=raw_input("Enter the car name: ")

        car_model=raw_input("Enter the car model: ")

        brand=model(car_model,car_name)

        if brand.car_name in cars1:

            print "1"

            print brand.car_name

#### here i am getting this error ######

            cars1[brand.car_name].append(brand.car_model)

        else:

            print "2"

            cars1[brand.car_name]=brand.car_model

            i+=1

    for key,values in cars1.items():

        print key+":"+values

        print cars1.items()