Python | Singleton

Singleton pattern: a design pattern. I.e., a class can generate a control target 
single mode widely used embodiment, such as music player, video, printer, etc.

Standard wording Singleton pattern 
even need to write a single case when the contents of the User class and paste past

 1 class User:
 2 
 3     __instance = None
 4     def __new__(cls, *args, **kwargs):
 5         if cls.__instance is None:
 6             cls.__instance = super().__new__(cls)
 7         return cls.__instance
 8 
 9 
10 u1 = User()
11 u2 = User()
12 print(id(u1))
13 print(id(u2))

 

Case:

Office printer 

requirements: 
a printer to serve everyone in an office to complete their print job 

analysis: 
Create three categories 
1. Printer: 1) add the task to the task queue to be printed in print; 2) completion of printing operation 
2. manager: the task to be added to the printer to print 
3. employees: Adds the task you want to print to the printer

 1 class Printer:
 2     __instance = None
 3     __pr_list = []
 4     def __new__(cls, *args, **kwargs):
 5         if cls.__instance is None:
 6             cls.__instance = super().__new__(cls)
 7         return cls.__instance
 8 
 9     @staticmethod
10     def add_task(pr_info):
11         Printer.__pr_list.append(pr_info)
12         # Printer.to_print () # wrong! Each time the task will be added before the table of contents in both print and then again 
13  
14      @staticmethod
 15      DEF to_print ():
 16          Print (Printer. __Pr_list )
 17  
18  
19  class Manager:
 20      @staticmethod
 21      DEF use_printer (Printer, info):
 22 is          printer.add_task (info)
 23 is  
24  
25  class Stuff:
 26 is      @staticmethod
 27      DEF use_printer (Printer, info):
 28          printer.add_task (info)
 29  
30  
31 is= Printer Printer ()
 32  
33 is Manager = Manager ()
 34 is manager.use_printer (Printer, " manager print " )
 35  
36 Stuff = Stuff ()
 37 [ stuff.use_printer (Printer, " employee print " )
 38 is  
39 printer.to_print ()