Python: Singleton Class

Singleton class

| Singleton is a creational design pattern, which ensures that only one object of its kind exists and provides a single point of access to it for any other code. (Refactoring Guru)

| A singleton class is a class that only allows creating one instance of itself. (Python Morsels)

How to make a Singleton in Python

There are a few different ways to create a singleton class in Python. Below is from an example from Python Morsel’s:

class Example:
    _self = None

    def __new__(cls):
        if cls._self is None:
            cls._self = super().__new__(cls)
        return cls._self

    def __init__(self):
        self.attribute1 = "I'm an attribute 1"

• __init__ method is the initializer method.

• By the time the __init__ method gets called, our class instance has already been constructed. That’s what the self argument in a method is:

  def __init__(self):
      ...
  

• The __new__ method is the constructor method. Its job is to do the actual work of constructing and then returning a new instance of our class:

  class Example:
  _self = None
  
  def __new__(cls):
      if cls._self is None:
          cls._self = super().__new__(cls)
      return cls._self
  

  • In the Example class, our constructor always returns the same instance.
• Since every instance of the Example class is the same, it means that we made a singleton class.

References

• Python Morsels: Singletons in Python
• Refactoring Guru: Singleton in Python