python如何判断变量是否为字符串

def add_key(self,keyname):

    self.keyname=keyname

    if  isinstance(self.keyname,str):

        self.num.append(self.keyname)

        return self.num

def update_list(self,str_1):  #不能定义python的默认参数

        self.str_1=str_1

        if type(self.str_1)==str:

            return self.num+self.str_1