python的类定义的属性如果是列表为什么要预先赋值一个空列表

Question

class Privileges():
	def __init__(self,privileges=[]):
		self.privileges=['can add post','can delete post','can ban users']
	def show_privileges(self):
		for privilege in self.privileges:
			print(" --"+privilege)
			
admin=Admin('bruce','lee')
admin.privileges.show_privileges()

请问为什么第二行的privileges后面要预先赋值一个空列表呢？否则就会报错。

class Privileges():
    def __init__(self,privileges):
        self.privileges = privileges
    def show_privileges(self):
        for privilege in self.privileges:
            print("Admin" + privilege)
admin = Privileges([' can add post',' can delete post',' can ban user'])
admin.show_privileges()

我知道赋予空列表是给一个默认值，但如果是上面这样编写，那第二行的privilegs为什么就不用预先赋予一个空列表呢？

Answer 1

你的第一个是直接将['can add post','can delete post','can ban users']赋值给了类的属性privileges，在实例化类时，因为有默认参数，所以你没传参也没报错啊，而第二个是将['can add post','can delete post','can ban users']当做privileges参数传入到了Privileges类中，这时候没有默认参数，你传参了所以不报错，不传参就会报错啊

 

这是没有默认值的，也就是说必须传入privileges参数，否则会报错：

class Privileges():
    def __init__(self,privileges):
        self.privileges=privileges
    def show_privileges(self):
        for privilege in self.privileges:
            print("Admin"+privilege)

admin = Privileges([' can add post',' can delete post',' can ban user'])
admin.show_privileges()

这是有默认值的，可以不传入参数：

class Privileges():
    def __init__(self,privileges=[]):
        self.privileges = privileges
    def show_privileges(self):
        for privilege in self.privileges:
            print("Admin" + privilege)
            
admin = Privileges()
admin.show_privileges()

也可以传入参数：

class Privileges():
    def __init__(self,privileges=[]):
        self.privileges = privileges
    def show_privileges(self):
        for privilege in self.privileges:
            print("Admin" + privilege)
            
admin = Privileges([' can add post',' can delete post',' can ban user'])
admin.show_privileges()