Object class, and__ new__，__ init__，__ setattr__，__ dict__

summary ：

1：__new__ For building instances of classes , and __init__ Used to initialize the instance

2：__setattr__ Actually, it's going to __dict__ Add a new key value pair

3：object Class is python The parent of all classes in , in other words python All classes in are inherited from object, So when defining a class , When there is no indication of its parent class , In fact, this class also has a parent class , Namely object. and __new__,__init__,__setattr__,__dict__ These are all object Class methods or properties , We customize the use of classes , If you use these , Is to object Rewrite of these methods .

4： Add ：__str__, __repr__

One ：__new__ and __init__

Reference from ：

(12 Bar message ) Python Medium __new__ and __init__ The difference between _ Yao Kai's blog of the former shock master -CSDN Blog _pythonnew and init The difference between (12 Bar message ) Python Class object What is it? _NickTheRock The blog of -CSDN Blog _object stay python What do you mean in English (12 Bar message ) Python Medium __new__ and __init__ The difference between _ Yao Kai's blog of the former shock master -CSDN Blog _pythonnew and init The difference between

When instantiating a class , It's actually calling __new__, Call again __init__ Of , And only __new__ Return an instance , Will call __init__, otherwise __init__ Method will not be called .

And __new__ The first parameter of the method is not self, It is cls, because self Reference instance , and __new__ When the function is called , Instance has not been created , So it can't be self, It is __new__(cls), Corresponding ,__new__ The instance returned by the method is

super（）.__new__(cls), That is to call the base class Object Of __new__ Method

Two ：__setattr__ and __dict__

__dict__ Contains all attribute key value pairs of a class , But classes and instances __dict__ There's a difference ： as follows , Example of __dict__ Include only the attributes you define , Second class __dict__ contain object Built-in properties

Reference from ：(12 Bar message ) Python in __dict__ usage _7Drunk The blog of -CSDN Blog _python in __dict__

  Again, __setattr__ and __dict__ The relationship between , about __setattr__ This built-in function , When defining attributes on a class , This function will be called automatically , As shown in the figure below ：

  Note that at this time ,self.__dict__[key] = value and object.__setattr__(self,key,value) It is equivalent. , The first is equivalent to modifying the class directly __dict__, The latter is done by calling the parent class object Of __setattr__ To modify the class __dict__; About __setattr__ For more detailed information about the function, please refer to ：

(12 Bar message ) Python Magic methods （ 3、 ... and ） __getattr__,__setattr__, __delattr___yusuiyu The blog of -CSDN Blog ___getattr__()

Add ： about __getattr__, This function is only used when calling properties that the class does not have , Class will automatically call this function , But it doesn't add this new attribute to __dict__, That is to say __getattr__ Not invoke __setattr__

  Again , For assigning new attributes to classes , It will only call __setattr__, Instead of calling __getattr__

 

  3、 ... and ：__str__,__repr__

a key ：

1： Both functions must have a return value , And the return value must be a string , And there's no __str__ Under the circumstances , Would call __repr__

2： Reference from ：

(12 Bar message ) python in %s And %r The difference between _fanzuifenzi_zsc The blog of -CSDN Blog _python in %r(12 Bar message ) python-- Basic knowledge points --__str__、__repr___Chasing__Dreams The blog of -CSDN Blog

• __str__ Trigger mode
  • prtin()
  • str()
  • format()
• __repr__ Trigger mode
  • In interactive mode, directly use the variable name of the reference object to view the object . That is, input the variable name under the terminal , Then go straight to enter  
  • repr(）

It is worth noting that s% Will pass variables into str（） function , and r% Will pass variables into repr

Reference from ：(12 Bar message ) Python in __str__ and __repr__ The difference between _Python Learners' blogs -CSDN Blog

(12 Bar message ) Python Class built-in methods __str__ , __repr___ The blog of little peach fan -CSDN Blog

(12 Bar message ) python in %s And %r The difference between _fanzuifenzi_zsc The blog of -CSDN Blog _python in %r

For a class , These two built-in functions are used to express objects , But we define a class , If not set __str__ perhaps __rper__ function , The source and memory address of the object are returned ：

  If you set __str__, Call it

  about __repr__ function , If there is no definition __str__, Should have called __str__ If you do, you will call __repr__：