Add singleton metaclass example

README.md

@@ -333,6 +333,20 @@ Useful if you want to share cached information to multiple objects.
 from typing import Any, Dict
 
 
+class SingletonMeta(type):
+ """Singleton metaclass implementation."""
+
+ def __init__(cls, cls_name: str, bases: tuple, namespace: dict):
+ cls.__instance = None
+ super().__init__(cls_name, bases, namespace)
+
+ def __call__(cls, *args, **kwargs):
+ if cls.__instance is None:
+ cls.__instance = super().__call__(*args, **kwargs)
+ return cls.__instance
+ return cls.__instance
+
+
 class Singleton:
  """Makes all instances as the same object."""
 

patterns/creational/singleton.py

@@ -1,9 +1,31 @@
 from typing import Any, Dict
 
 
+class SingletonMeta(type):
+ """Singleton metaclass implementation."""
+
+ def __init__(cls, cls_name: str, bases: tuple, namespace: dict):
+ cls.__instance = None
+ super().__init__(cls_name, bases, namespace)
+
+ def __call__(cls, *args, **kwargs):
+ if cls.__instance is None:
+ cls.__instance = super().__call__(*args, **kwargs)
+ return cls.__instance
+ return cls.__instance
+
+
+class Single(metaclass=SingletonMeta):
+ """Singleton object."""
+
+ pass
+
+
 class Singleton:
  """Makes all instances as the same object."""
 
+ _instance: "Singleton"
+
  def __new__(cls) -> "Singleton":
  if not hasattr(cls, "_instance"):
  cls._instance = super().__new__(cls)
@@ -29,6 +51,8 @@ class Bar:
  pass
 
 
+print(Single() is Single())
+
 singleton_one: Singleton = Singleton()
 singleton_two: Singleton = Singleton()