Here I have defined immutable class str. In new method I am changing the values of instances such as "hello" to uppercase. Why should we do it using new when we can define upper in init ?

class Upperstr(str):
    def __new__(cls,value=""):
        print(cls)
        print(value)
        return str.__new__(cls,value.upper())

    # def __init__(self,m1):
    #     self.m1 = m1.upper()
    
u = Upperstr("hello")
print(u)

New is used to create class instances. What are the other uses of new method?

CodePudding user response：

New is used to create class instances. What are the other uses of new method?

You can use __new__ to implement singleton pattern (where pattern must be understand as thing described in Design Patterns: Elements of Reusable Object-Oriented Software), take look at example provided by geeksforgeeks.org of Classic Singleton

class SingletonClass(object):
  def __new__(cls):
    if not hasattr(cls, 'instance'):
      cls.instance = super(SingletonClass, cls).__new__(cls)
    return cls.instance
   
singleton = SingletonClass()
new_singleton = SingletonClass()
 
print(singleton is new_singleton)
 
singleton.singl_variable = "Singleton Variable"
print(new_singleton.singl_variable)

CodePudding user response：

new is basically a standard Python method that is invoked before init when a class instance is created. For further information, see the python new manual: https://docs.python.org/3/reference/datamodel.html#object.__new__

There is not other direct usage of it.

Regarding the difference between Init/New: The constructor function in python is called new and init is the initializer function. According to the Python documentation, new is used to control the creation of a new instance, whereas init is used to handle its initialization.