Singleton in Python
Singleton is a creational design pattern, which ensures that only one object of its kind exists and provides single point of access to it for any other code.
Singleton has almost the same pros and cons as global variables. Although they’re super-handy, they break modularity of your code.
You can just use a class, which depends on Singleton, in some other context. You’ll have to carry the Singleton class as well. Most of the time, this limitation comes up during the creation of unit tests.
Learn more about Singleton
Usage of the pattern in Python
Usage examples: A lot of developers consider the Singleton pattern an antipattern. That’s why its usage is on the decline in Python code.
Identification: Singleton can be recognized by a static creational method, which returns same cached object.
It’s pretty easy to implement a sloppy Singleton. You just need to hide constructor and implement a static creational method.
Same class behaves incorrectly in a multithreaded environment. Multiple threads can call creational method simultaneously and get several instances of Singleton class.
main.py: Conceptual Example
from __future__ import annotations
from typing import Optional
The Singleton class defines the `getInstance` method that lets clients
access the unique singleton instance.
_instance: Optional<a href="/design-patterns/singleton">Singleton</a> = None
def __init__(self) -> None:
if Singleton._instance is not None:
raise ReferenceError("Cannot instantiate a singleton class.")
Singleton._instance = self
def get_instance() -> Singleton:
The static method that controls the access to the singleton instance.
This implementation let you subclass the Singleton class while
keeping just one instance of each subclass around.
if not Singleton._instance:
Finally, any singleton should define some business logic, which can
be executed on its instance.
if __name__ == "__main__":
# The client code.
s1 = Singleton.get_instance()
s2 = Singleton.get_instance()
if id(s1) == id(s2):
print("Singleton works, both variables contain the same instance.")
print("Singleton failed, variables contain different instances.")
Singleton works, both variables contain the same instance.
Singleton in Other Languages