Recently, I went through some interviews from big name companies, such as Tencent and Bytedances. In the following posts, I would like to share some knowledge points, one for noting and another for your information.
What is Singleton
Singleton pattern is a useful software design pattern that restricts the instantiation of a class to oe object. When only one exactly object is needed to use across the system, it becomes useful since it reduces the instantiation time.
A singleton class can and only can have one instance, and it should provide other objects a way to access the instance.
When you want to control the number of instances, and/or save system resources, you should use it.
- No need for frequently creating and destorying instances
- Prevent from multiple occupation of resources, like file access
How to make it
Simple explanation: Let the class itself responsible for controlling its instantiation.
The implementation of the singleton pattern shall,
- provide access to the instance;
- ensure that only one instance exists
Typically, this is done by,
- declaring all constructors to be private
- providing a static method that returns a reference to the instance.
We should be aware that, if multi-threads is used in project, a double check (synchronized lock) should be used. Otherwise, if multiply threads request the instance, more than one instance may be instantized.
In Wikipedia, many examples are provided, but I still want to share some other examples in Python.
By nature, all modules are singletons because Python will cache the module initialization (the byte-compiled .pyc files)
A program doesn't run any faster when it is read from a ‘.pyc’ or ‘.pyo’ file than when it is read from a ‘.py’ file; the only thing that's faster about ‘.pyc’ or ‘.pyo’files is the speed with which they are loaded. When a script is run by giving its name on the command line, the bytecode for the script is never written to a ‘.pyc’ or ‘.pyo’ file. Thus, the startup time of a script may be reduced by moving most of its code to a module and having a small bootstrap script that imports that module. It is also possible to name a ‘.pyc’ or ‘.pyo’file directly on the command line.
So if we want to quickly make a singleton, modules are your good friend. For example…
Save it as mysingleton.py, and import it using…
from mysingleton import my_singleton
A decorator is the name used for a software design pattern. Decorators dynamically alter the functionality of a function, method, or class without having to directly use subclasses or change the source code of the function being decorated.
Here we will use a decorator to decorate the singleton class.
from functools import wraps
__new__ is a special method to control the instantiation of instances. We can utilize it to make our class singleton.
Here we make a superclass for inheritance.
To be honest, metaclass is the most difficult-to-understand concept in Python… If you want to learn it, check https://stackoverflow.com/questions/100003/what-are-metaclasses-in-python.
A metaclass is the class of a class. Think about instances of class, we need to class to create an instance. What if we want to create a class? Bingo, we need a metaclass to create a class. Using a metaclass, we can achieve…
- intercept the creation of class
- modify the definition of a class
- return the modified class
I won’t go too far here, and only some code will be here. You will find it similar to the previous one :)
Use modules, which should be enough for most cases.
In Java, since I know nothing about it, I recommend you to refer to this post (in Chinese).
For other languages, use Google :)
- Wikipedia contributors. (2018, March 29). Singleton pattern. In Wikipedia, The Free Encyclopedia. Retrieved 18:29, April 15, 2018, from https://en.wikipedia.org/w/index.php?title=Singleton_pattern&oldid=833049720