(本文是我自己的理解)
类属性: class本身和实例化的对象都有
实例属性: 只有实例化的对象有，class本身(未实例化之前)是没有的
理由:
类属性直接写于class下，例如
class A(): attr1 = "I‘m class attr1" print(A.attr1) print(dir(A)) ‘‘‘ I‘m class attr1 [‘__class__‘, ‘__delattr__‘, ‘__dict__‘, ‘__dir__‘, ‘__doc__‘, ‘__eq__‘, ‘__format__‘, ‘__ge__‘, ‘__getattribute__‘, ‘__gt__‘, ‘__hash__‘, ‘__init__‘, ‘__init_subclass__‘, ‘__le__‘, ‘__lt__‘, ‘__module__‘, ‘__ne__‘, ‘__new__‘, ‘__reduce__‘, ‘__reduce_ex__‘, ‘__repr__‘, ‘__setattr__‘, ‘__sizeof__‘, ‘__str__‘, ‘__subclasshook__‘, ‘__weakref__‘,
‘attr1‘] ‘‘‘
实例属性需要前缀, 例如
class A(): def __init__(self): self.attr2 = "I‘m attr2"
在实例化一个对象的时候，第一个调用的是构造函数__new__, 然后在调用__init__初始化对象
class A(): def __new__(cls, *args, **kwargs): """do something""" print("I‘m new") return super(A, cls).__new__(cls, *args, **kwargs) def __init__(self, *args, **kwargs): print("I‘m init") #do something self.attr1 = ‘attr1‘ a = A() # I‘m new # I‘m init
如果__new__不返回父级的__new__方法(最上层当然是object),就不会调用__init__
class A(): def __new__(cls, *args, **kwargs): """do something""" print("I‘m new") # return super(A, cls).__new__(cls, *args, **kwargs) def __init__(self, *args, **kwargs): print("I‘m init") #do something self.attr1 = ‘attr1‘ a = A() # I‘m new
在对象没有实例化的时候，是没有self这一说的..所以依靠self生成的函数都不存在。
所以实例属性不会在类中，只会在实例中，而类属性可以在实例中
class A(): attr1 = "I‘m attr1" def __init__(self): self.attr2 = "I‘m attr2" print(A.attr1) # I‘m attr1 print(A.attr2) # AttributeError: type object ‘A‘ has no attribute ‘attr2‘ print(dir(A)) #[‘__class__‘, ‘__new__‘... ‘__weakref__‘, ‘attr1‘] a = A() print(a.attr1) # I‘m attr1 print(a.attr2) # I‘m attr2 print(dir(a)) #[‘__class__‘, ...‘__setattr__‘, ‘__sizeof__‘, ‘__str__‘, ‘__subclasshook__‘, ‘__weakref__‘, ‘attr1‘, ‘attr2‘]
