【Python】类属性和实例属性的差异

(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']