Next
1. Create Class & Object2. Instance vs Class Variables3. Constructors in Python Classes4. Single Inheritance5. Multiple Inheritance6. Multilevel Inheritance7. Polymorphism with Methods8. Method Overloading Simulation9. Encapsulation with Getters/Setters10. Abstraction with ABC11. Operator Overloading12. Custom Iterator13. Class Methods14. Static Methods15. Composition Example16. Aggregation Example17. Private Variables18. Magic Methods Overview19. Object Cloning20. Custom Exceptions21. Class Decorator22. Prototype Pattern23. Singleton Pattern24. MRO (Method Resolution Order) Demo25. Polymorphism (Duck Typing)

Singleton Pattern in Python

Only one instance. Call the constructor twice, get the same object back. People overuse this.

IntermediateObject-Oriented ProgramsExample 23 of 25
singleton-pattern.py
1# Program to implement a simple Singleton pattern
2

3class Singleton:
4    _instance = None
5

6    def __new__(cls, *args, **kwargs):
7        if cls._instance is None:
8            cls._instance = super().__new__(cls)
9        return cls._instance
10

11s1 = Singleton()
12s2 = Singleton()
13

14print(s1 is s2)

Output

True

What's going on

Overriding __new__ ensures that only one instance of Singleton is ever created.

After this program

If the idea is still fuzzy, read Day 11: Classes and Objects in the Python course. For a short check, try the Python quiz (300 questions). Stay with all 25 object-oriented programs, or see every topic on Python exercises.

PreviousPrototype PatternNextMRO (Method Resolution Order) Demo