python - 为什么为第二个实例调用保留默认参数？

Question

当我执行下面的代码时，第一次打印名称值时，它会打印正确的名称，即“John”。

但是当我创建第二个空实例时，它仍然会打印名称“John”。预期的输出应该是，它只会打印“空名称”，因为friend_2使用了一个新的空实例。

class User:
    def __init__(self,name ="Empty name"):
        self.__name = name
        
    def get_name(self):
        return self.__name
        
    def set_name(self,word):
        self.__name = word        
        
class Friend:
    def __init__(self,user = User()):
        self.__user = user
        
    def get_user(self):
        return self.__user.get_name()
    
    def set_n(self,name):
         self.__user.set_name(name)    
        

   


friend_1 = Friend()
# Only friendd_1 instance is being set to name John
friend_1.set_n("John") 

name_1 = friend_1.get_user()  # expected as we set John to friend_1 instance

friend_2 = Friend()    # use new User empty instance

name_2 = friend_2.get_user()  # This value is also John, but it should print "Empty value" as we are using default argument.

标签： pythondefault

Answer 1

正如评论中所述，可变默认参数是一个常见的问题。

class Friend:
    def __init__(self, user = None):
        # Default to None instead and then create the object during initialization 
        self.__user = user if user else User()