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This article mainly introduces "what are the special methods in Python". In daily operation, I believe many people have doubts about what special methods there are in Python. The editor consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful for you to answer the doubts about "what special methods are there in Python?" Next, please follow the editor to study!

Catalogue

1. Overview

2. Common special methods and implementation

2.1 _ len__ ()

2.2 _ _ str__ ()

2.3 _ _ iter__ ()

2.42.4 _ _ getitem__ ()

2.5 _ _ new__ ()

2.6 implement singleton mode using _ _ new__ ()

2.7 _ _ call__ ()

2.8 percent enterprising _ ()

2.9 _ add__ ()

2.10 _ _ del__ ()

1. Overview

Special methods (magic methods) in python are called by the python interpreter, we don't need to call them ourselves, we use built-in functions to use them. For example, after the special method _ _ len__ () is implemented, we only need to use the len () method; there are also some special method calls that are implicit, such as: for i in x: behind the utility is the built-in function iter (x).

Some common special methods and implementations are described below. Explain by implementing a class

2. Common special methods and implementation of 2.1 _ len__ ()

The quantity is generally returned, using the len () method call. The len () function can also be used inside _ _ len__ ()

Class Zarten (): def _ init__ (self, age): self.age = age self.brother = ['zarten_1',' zarten_2'] def _ len__ (self): return len (self.brother) # you can directly use len () # return self.agez = Zarten (18) print (len (z)) 2.2 _ str__ ()

The string representation of the object is basically the same as _ _ repr__ (), with a slight difference in:

_ _ str__ () is for end users, while _ _ repr__ () is for developers, debugging, logging, etc.

On the command line, after implementing _ _ str_ (), entering the object name directly displays the object's memory address, while implementing _ _ repr__ () has the same effect as print (object).

If both of them are implemented, _ _ str_ () will be called. Generally, at least _ _ repr__ () will be implemented in the class.

Class Zarten (): def _ _ repr__ (self): return'my name is Zarten_1' def _ _ str__ (self): return'my name is Zarten_2'z = Zarten () print (z) 2.3 _ iter__ ()

Returns an iterable object, usually used with _ _ next__ ()

Determine whether an object is: iterable object from collections import Iterable

From collections import Iterableclass Zarten (): def _ _ init__ (self Brother_num): self.brother_num = brother_num self.count = 0 def _ iter__ (self): return self def _ next__ (self): if self.count > = self.brother_num: raise StopIteration else: self.count + = 1 return 'zarten_' + str (self.count) zarten = Zarten (5) print (' is iterable:', isinstance (zarten) Iterable)) # determine whether it is an iterable object for i in zarten: print (I) 2.4 _ _ getitem__ ()

This special method returns data, can also replace the _ _ iter_ () and _ _ next__ () methods, and can also support slicing

Class Zarten (): def _ _ init__ (self): self.brother = ['zarten_1','zarten_2','zarten_3','zarten_4','zarten_5',] def _ getitem__ (self, item): return self.brother [item] zarten = Zarten () print (zarten [2]) print (zarten [1:3]) for i in zarten: print (I) 2.5 _ new__ ()

_ _ new__ () is used to construct an instance of a class. The first parameter is cls, which is not normally used. And _ _ init__ () is used to initialize the instance, so _ _ new__ () is executed before _ _ init___ ().

If _ _ new__ () does not return, no object will be created and _ _ init___ () will not be executed

If _ _ new__ () returns an instance of another class, _ _ init___ () will not execute either.

Purpose: singleton mode can be implemented using _ _ new___ ()

Class Zarten (): def _ new__ (cls, * args, * * kwargs): print ('_ new__') return super (). _ new__ (cls) def _ init__ (self, name) Age): print ('_ init__') self.name = name self.age = age def _ repr__ (self): return 'name:% s age:%d'% (self.name,self.age) zarten = Zarten (' zarten', 18) print (zarten)

2.6.Use _ _ new__ () to implement singleton pattern class Zarten (): _ singleton = None def _ _ new__ (cls, * args, * * kwargs): print ('_ new__') if not cls._singleton: cls._singleton = super (). _ new__ (cls) return cls._singleton def _ init__ (self, name) Age): print ('_ init__') self.name = name self.age = age def _ repr__ (self): return 'name:% s age:%d'% (self.name,self.age) zarten = Zarten (' zarten', 18) zarten_1 = Zarten ('zarten_1', 19) print (zarten) print (zarten_1) print (zarten_1 = = zarten)

2.7 _ _ call__ ()

After implementation, the object can become a callable object, which can be called like a function. For example, custom functions, built-in functions, and classes are all callable objects. You can use callable () to determine whether they are callable objects.

Class Zarten (): def _ _ init__ (self, name, age): self.name = name self.age = age def _ _ call__ (self): print ('name:%s age:%d'% (self.name, self.age)) z = Zarten (' zarten', 18) print (callable (z)) z () 2.8 minutes enterprising

A context manager class must implement these two special methods: _ _ enter_ () and _ _ exit__ () are called using the with statement.

Use _ _ enter__ () to return the object and _ _ exit__ () to close the object

Class Zarten (): def _ init__ (self, file_name, method): self.file_obj = open (file_name, method) def _ enter__ (self): return self.file_obj def _ exit__ (self, exc_type, exc_val, exc_tb): self.file_obj.close () print ('closed') with Zarten (' e:\\ test.txt' 'r') as f: r = f.read () print (r) 2.9 _ _ add__ ()

Addition operator overloading and _ _ radd__ () reverse operator overloading

When an object is added, it will first look for _ _ add__ () on the left side of "+", and look for _ _ radd__ () on the right side of "+" if it cannot be found.

Class Zarten (): def _ init__ (self, age): self.age = age def _ add__ (self, other): return self.age + other def _ radd__ (self, other): return self.age + otherz = Zarten (18) print (z + 10) print (20 + z) 2.10 _ del__ ()

Called at the end of the object's life cycle, which is equivalent to a destructor

Class Zarten (): def _ init__ (self, age): self.age = age def _ del__ (self): print ('_ del__') z = Zarten (18)

Summary list of special (magic) methods

At this point, the study of "what are the special methods in Python" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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