Python： Magic methods (__getitem__、__len__ And other methods that contain double underscores ) An introduction to the 、 Detailed introduction to use cases
Catalog
Magic methods Magic Method An introduction to the
(1)、 For example, to understand Magic Method
Common magic method use cases
1、 Common methods
(1)、__init__： Initialization method
(2)、__len__()： Returns the number of elements in the container
2、 Comparison operator
(1)、__lt__(self, other)： Define the behavior of the less than sign
3、 Arithmetic operator
(1)、__add__(self, other)： The act of defining addition ：+
4、 Incremental assignment operation
(1)、__iadd__(self, other)： Define the behavior of assignment addition ：+=
5、 Unary operators
(1)、__pos__(self)： Define positive behavior ：+x
6、 Container type
(1)、_getitem__()： Return the value of the key
Magic Method, Magic method , seeing the name of a thing one thinks of its function , These methods are powerful , Full of magic , Can let you realize many functions . The magic method is python Built-in methods , No active call required , The purpose of existence is to to python The interpreter of , Almost every magic method has a built-in function , Or operator , When we use these functions or operators on this object, we will call the corresponding magic methods in the class , It can be understood as Rewrite these python Built in functions for .
stay Python in , image __getitem__ This method consists of two double underscores , Unification is called the magic method . The magic method is to give python The interpreter uses . Magic methods are methods belonging to classes , All of them operate on classes . That is, the method can be accessed without instantiating the class , meanwhile , All instantiated objects can access this method .
For example, when using len(function) when , It's actually called object.__len__ Method . But in the use of function['key'] To access the element , It's actually calling theta object.__getitem__(key) Method .
Use __getitem__ and __len__ Method , We can implement an iteration and access to custom data types .
Constructors , Initialization method called when an instance is created
class Test_init:
def __init__(self):
print(" One of the magic methods __init__ Method ")
print(Test_init())
To make... For an object instance of a class len() Function executed successfully , Must be defined in the class __len__(). If there is no definition __len__(), Then input len(c1) It will prompt you to make mistakes , Obviously the reason for the mistake is Class01 There is no len() The definition of . because len() You must receive the properties of a specific instantiated object , If change to len(c1.values) Can also successfully execute ！
# __ len__()： Returns the number of elements
class Test_len(object):
def __init__(self, *args):
self.names = args
def __len__(self):
print(" One of the magic methods __len__ Method ")
return len(self.names)
res02 = Test_len('Bob', 'Alice', 'Tom')
print(len(res02))x < y call x__lt__(y)
If this is defined in the class __getitem__ Method , Then its instance object （ Is assumed to be p）, It can be like this p[key] Value . When instance objects do p[key] Operation time , Will call the methods in the class __getitem__.__getitem __ You can let objects implement iterative functions , In this way, the data can be called continuously .
class Test_getitem:
def __init__(self,id):
self.id=id
# , Its instance object P, We can use P[key] Formal value
def __getitem__(self, item): # When instance objects do P[key] Operation time , It will call __getitem__() Method
print(" One of the magic methods __getitem__ Method ")
return self.id
res = Test_getitem('This is id')
print(res.id)
print(res[0]) # When instance objects do P[key] Operation time , It will call __getitem__() Method
print(res['0000'])
