The Singleton pattern is a creational design pattern that restricts the instantiation of a class to a single object. In Python, this is commonly used for shared resources like database connections, loggers, or configuration settings.
Singleton ensures:
- Only one object instance exists
- Global shared access point is available
Basic Singleton Example
class Singleton:
_instance = None
def __new__(cls):
if cls._instance is None:
cls._instance = super().__new__(cls)
return cls._instance
obj1 = Singleton()
obj2 = Singleton()
print(obj1 is obj2)
⚠️ Problem in Multithreading
Two threads may create multiple instances simultaneously.
Thread-Unsafe Singleton
class Singleton:
_instance = None
def __new__(cls):
if cls._instance is None:
cls._instance = super().__new__(cls)
return cls._instance
✅ Thread-Safe Singleton Using Lock
import threading
class Singleton:
_instance = None
_lock = threading.Lock()
def __new__(cls):
with cls._lock:
if cls._instance is None:
cls._instance = super().__new__(cls)
return cls._instance
Singleton Using Decorator
def singleton(cls):
instances = {}
def wrapper(*args, **kwargs):
if cls not in instances:
instances[cls] = cls(*args, **kwargs)
return instances[cls]
return wrapper
@singleton
class Database:
pass
db1 = Database()
db2 = Database()
print(db1 is db2)
Singleton Using Metaclass
class SingletonMeta(type):
_instances = {}
def __call__(cls, *args, **kwargs):
if cls not in cls._instances:
cls._instances[cls] = super().__call__(*args, **kwargs)
return cls._instances[cls]
class Logger(metaclass=SingletonMeta):
pass
obj1 = Logger()
obj2 = Logger()
print(obj1 is obj2)
📊 Best Singleton Approach
