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【Python】【Magic Method】(I) Constructing and Initialization

Popularity：768 ℃/2025-03-25 10:12:48

The magic method explained in this article:

  • __new__

  • __init__

  • __del__

  • __repr__

  • __format__

  • __bytes__

1. __new__

In Python,__new__Method is a special method used to control the creation process of an object. understand__new__The mechanism of methods is very helpful for mastering Python's class and object models. The following is the right__new__Detailed introduction to the method:

1.1 Basic Concept

  • __new__method: This is a class method that is usually used to create and return a new instance of a class. It is called automatically when instantiating an object,__init__The method is executed earlier.
  • Return value：__new__The method must return an instance object, usually an instance of the current class. If returnNone, it will not be called__init__method.

1.2 Method Signature

__new__The basic signature of the method is as follows:

class MyClass:
     def __new__(cls, *args, **kwargs):
         # Create and return a new instance
         instance = super().__new__(cls)
         Return instance
  • clsparameter: Represents a reference to the current class, similar toselfparameter.
  • *argsand**kwargs: represents the parameter passed to the class constructor.

1.3 Common usage

1.3.1 Basic Instantiation

When calling the class constructor (i.e., the class name is parenthesed),__new__The method will be called. For example:

class MyClass:
    def __new__(cls, *args, **kwargs):
        print("MyClass __new__ method called")
        return super().__new__(cls)

    def __init__(self, value):
        print("MyClass __init__ method called")
         = value

obj = MyClass(10)

Output:

MyClass __new__ method called
MyClass __init__ method called
1.3.2 Inheriting immutable types

When inheriting immutable types (e.g.int、str、tupleWhen you are, you usually need to rewrite it__new__Methods to create objects, because these types are immutable and cannot be__init__Modify their values ​​in the method. For example:

class MyInt(int):
     def __new__(cls, value):
         print("MyInt __new__ method called")
         return super().__new__(cls, value)

 num = MyInt(10)
 print(num) # Output: 10

1.3.3 Singleton Mode

__new__Methods can be used to implement singleton pattern, i.e. ensure that a class has only one instance. For example:

class Singleton:
     _instance = None

     def __new__(cls, *args, **kwargs):
         if not cls._instance:
             cls._instance = super().__new__(cls)
         return cls._instance

     def __init__(self, value):
          = value

 obj1 = Singleton(10)
 obj2 = Singleton(20)
 print(obj1 is obj2) # Output: True
 print() # Output: 10
 print() # Output: 10
1.3.4 Metaclass

In metaclasses,__new__Methods can be used to control the creation process of a class. For example:

class Meta(type):
    def __new__(cls, name, bases, dct):
        print("Meta __new__ method called")
        return super().__new__(cls, name, bases, dct)

class MyClass(metaclass=Meta):
    pass

obj = MyClass()

Output:

Meta __new__ method called

1.4 with__init__Differences in methods

  • __new__method: Responsible for creating an instance object, usually returning an instance.
  • __init__method: Responsible for initializing the instance object and does not return any value.

1.5 Notes

  • Return value：__new__The method must return an instance object. If returnNone, it will not be called__init__method.
  • Call order：__new__Method in__init__The method is called before.
  • Class Methods：__new__A method is a class method, and the first parameter is the class itself (cls), not instances (self）。

Through understanding__new__How methods work and common usages, you can better control the creation process of objects and implement more complex class behaviors.

2. __init__

sure!__init__Methods are a special method of classes in Python and are usually used as a constructor for classes. When creating an instance of a class,__init__The method will be called automatically. The following is correct__init__Detailed introduction to the method:

1. Basic concepts

  • Constructor：__init__The main function of the method is to initialize the instance of the class. When a new object is created,__init__The method will be called.
  • Automatic call: You don't need to call it explicitly__init__method, it will be called automatically when object is created.
  • parameter：__init__Methods can accept parameters, which can be passed in when the object is created.

2. Basic syntax

class ClassName:
     def __init__(self, param1, param2, ...):
         self.attribute1 = param1
         self.attribute2 = param2
         # Other initialization codes

3. Example

class Person:
     def __init__(self, name, age):
          = name
          = age

 # Create an instance of Person class
 p1 = Person("Alice", 30)

 # Access instance properties
 print() # Output: Alice
 print() # Output: 30

4. Initialize multiple instances

class Person:
     def __init__(self, name, age):
          = name
          = age

 # Create multiple instances of Person classes
 p1 = Person("Alice", 30)
 p2 = Person("Bob", 25)

 print() # Output: Alice
 print() # Output: 30
 print() # Output: Bob
 print() # Output: 25

5. Default parameters

You can__init__The default values ​​are provided for parameters in the method, so that these parameters can be not passed in when creating the object.

class Person:
     def __init__(self, name, age=18):
          = name
          = age

 # Create an instance of Person class
 p1 = Person("Alice", 30)
 p2 = Person("Bob") # Use default age 18

 print() # Output: Alice
 print() # Output: 30
 print() # Output: Bob
 print() # Output: 18

6. Initialize complex objects

__init__Methods can contain more complex initialization logic, such as creating other objects, calling other methods, etc.

class Person:
     def __init__(self, name, age):
          = name
          = age
         self.initialize_details()

     def initialize_details(self):
          = f"{} is {} years old."

 # Create an instance of Person class
 p1 = Person("Alice", 30)

 print() # Output: Alice is 30 years old.

7. Inherited__init__method

In inheritance, subclasses can override the parent class's__init__Methods can also call the parent class__init__method.

class Person:
     def __init__(self, name, age):
          = name
          = age

 class Student(Person):
     def __init__(self, name, age, student_id):
         super().__init__(name, age) # Call the __init__ method of the parent class
         self.student_id = student_id

 # Create an instance of the Student class
 s1 = Student("Alice", 20, "12345")

 print() # Output: Alice
 print() # Output: 20
 print(s1.student_id) # Output: 12345

8. Things to note

  • Don't abuse it__init__method:Although__init__Methods can contain complex logic, but should be kept as concise as possible to avoid excessive complexity.
  • Calling the parent class__init__method: Rewrite in subclass__init__When a method, if you need to initialize the properties of the parent class, the parent class should be called__init__method.

Hope these contents help you understand better__init__method. If you have any specific questions or need further explanation, feel free to let me know!

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