How to Implement a Singleton Class in Python

This code demonstrates a classic Singleton pattern in Python by overriding __new__ to ensure only one instance of the class is created, even when instantiated multiple times.

Easy Python 3.9+ Aug 9, 2026 OOP & classes 15 views 0 copies

singleton class oop design-pattern

Python code

18 lines

Python 3.9+

class Singleton:
    _instance = None

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

    def __init__(self):
        self.value = 0


if __name__ == "__main__":
    s1 = Singleton()
    s2 = Singleton()
    s1.value = 42
    print(s1 is s2)
    print(s2.value)

Output

stdout

True
42

How it works

The Singleton class overrides __new__ to control instance creation. A class attribute _instance stores the single instance, initially None. When creating an instance, if _instance is None, a new instance is created and assigned; otherwise, the existing instance is returned. This ensures s1 and s2 point to the same object, so setting value on s1 is reflected in s2. The __init__ method still runs on each instantiation, so in practice you might guard state initialization to avoid overwriting.

Common mistakes

Forgetting that `__init__` runs every time, which can reset instance state
Not using thread-safe locking in multi-threaded environments
Assuming class inheritance will catch the singleton behavior without overriding again

Variations

Use a decorator to wrap the class and cache instances
Use a metaclass to enforce singleton behavior across subclasses

Real-world use cases

Managing a single database connection pool across an application to avoid resource leaks.
Sharing a global configuration object that is loaded once and accessed everywhere.
Implementing a central logging service to ensure consistent log formatting and single file handle.

How to Implement a Singleton Class in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from OOP & classes

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Tutorials

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