Differences between the "Python" class properties and the instance properties

(This article is my own understanding)

Class Properties: Both the class itself and the instantiated object have

Instance properties: Only instantiated objects have, class itself (before not instantiated) is not

Reason:

Class attributes are written directly under class, for example

class A ():     " 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 '] " "

Instance properties require a prefix, such as

class A ():     def __init__ (self):         " I ' m attr2 "

When instantiating an object, the first call is the constructor __new__, and then the __init__ initialization object is called

classA ():def __new__(CLS, *args, * *Kwargs):"""Do something"""        Print("I ' m new")        returnSuper (A, CLS).__new__(CLS, *args, * *Kwargs)def __init__(Self, *args, * *Kwargs):Print("I ' m init")        #Do somethingSELF.ATTR1 ='attr1'a=A ()#I ' m new#I ' m init

If __new__ does not return the parent's __new__ method (the topmost of course is object), the __init__ is not called

classA ():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 somethingSELF.ATTR1 ='attr1'a=A ()#I ' m new

When the object is not instantiated, there is no self to say. So the function that relies on self does not exist.

So instance properties are not in the class, only in the instance, and class properties can be in the instance

classA (): Attr1="I ' m attr1"    def __init__(self): SELF.ATTR2="I ' m attr2"Print(A.ATTR1)#I ' m attr1Print(A.ATTR2)#Attributeerror:type object ' A ' has no attribute ' attr2 'Print(dir (A))#[' __class__ ', ' __new__ ' ... ' __weakref__ ', ' attr1 ']A =A ()Print(A.ATTR1)#I ' m attr1Print(A.ATTR2)#I ' m attr2Print(Dir (a))#[' __class__ ', ... ' __setattr__ ', ' __sizeof__ ', ' __str__ ', ' __subclasshook__ ', ' __weakref__ ', ' attr1 ', ' attr2 ']

Python class properties and instance properties are different