Dp Singleton

In this tutorial, you'll learn about How to Fix Singleton Errors. We cover key concepts, practical examples, and best practices.

Fix singleton errors when thread safety not ensured or new bypasses instance check.

Quick Fix

Wrong

class Singleton:
    _inst = None
    def __init__(self):
        pass
    @classmethod
    def get(cls):
        if cls._inst is None:
            cls._inst = cls()
        return cls._inst

Race condition on first access. Two threads may create two instances.

Right

import threading
class Singleton:
    _inst = None; _lock = threading.Lock()
    def __new__(cls):
        if cls._inst is None:
            with cls._lock:
                if cls._inst is None:
                    cls._inst = super().__new__(cls)
        return cls._inst

s1=Singleton(); s2=Singleton(); s1 is s2 -> True. Thread-safe double-checked locking.

Prevention

Use new to control instance creation. Double-checked locking with mutex.

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