What is the constructor of python class

Python magic method: __init__() method, called the constructor method. The magic of the __init__() method is that as long as an object is instantiated, this method is automatically called when the object is created. Parameters can be passed in when instantiating an object. These parameters will be automatically passed into the __init__() method. You can customize the initialization operation of the object by overriding this method.

Summary: The __init__() method is a special method that will be called when the object is instantiated. The function of the __init__() method is initialization.

Example 1:

class Myclass:
    i = 123
    def __init__(self,friend):
        self.friend = friend
 
    def func(self,myself):    #带有参数的类方法
        self.myself = myself
        return self.myself +"say hello to " + self.friend
 
use_class = Myclass("xiaoming")
print("调用类的属性：",use_class.i)
print("调用类的方法:", use_class.func("xiaohong"))

Output result:

#调用类的属性： 123
#调用类的方法: xiaohongsay hello to xiaoming

Related recommendations: "Python Video Tutorial"

1. When defining a class in python without defining the __init__() method, the system will automatically call the default constructor (parameterless __init__() method) when the class is instantiated. In addition, __init__( ) method can have parameters, and the parameters are passed to the instantiation operation of the class through __init__()

2. Multiple constructors (__init__()) can be defined in a class, but only when instantiating a class Instantiate the last constructor, that is, the later constructor will overwrite the previous constructor and need to be instantiated according to the form of the last constructor (the parameters are consistent with the parameters of the last constructor). Therefore, it is best to define only one constructor method in a class

3. Initializing the instance includes: defining and initializing instance attributes or calling some methods of the class.

4. The constructor can have various parameters other than self (keyword parameters, default parameters, using tuples to collect parameters, using dictionaries to collect keyword parameters, etc.), which can be used when instantiating the class. , passing in the corresponding actual parameters.

When an object in a class is created, the constructor is called immediately.

Usage of constructor method init:

class FooBar:
    def __init__(self):
        self.somevar = 42
f = FooBar()
print f.somevar
（这里注意f = FooBar(),要带括号）

Output result:

42

Construction with parameters

class FooBar:
    def __init__(self,value=42):
        self.somevar = value
f = FooBar("this is a constructor argument")
print f.somevar

Output result:

this is a constructor argument

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