Singleton in python

Python/1-inner_class.py

@@ -0,0 +1,29 @@
+class Singletone:
+
+    class __Singletone:
+        def __init__(self, arg):
+
+            self.val = arg
+        def __str__(self):
+            return repr(self) + self.val
+
+    instance = None
+
+    def __init__(self, arg):
+        if not Singletone.instance:
+            Singletone.instance = Singletone.__Singletone(arg)
+        else:
+            Singletone.instance.val = arg
+
+    def __getattr__(self, name):
+        return getattr(self.instance, name)
+
+
+a = Singletone('first')
+print(a)
+b = Singletone('second')
+print(b)
+c = Singletone('third')
+print('a: ', a)
+print('b: ', b)
+print('c: ', c)

Python/2-new_method.py

@@ -0,0 +1,37 @@
+class Singletone(object):
+
+    class __Singletone:
+        def __init__(self):
+            self.val = None
+
+        def __str__(self):
+            return repr(self) + self.val
+
+    instance = None
+
+    def __new__(cls):
+        if not Singletone.instance:
+            Singletone.instance = Singletone.__Singletone()
+        return Singletone.instance
+
+    def __getattr__(self, name):
+        return getattr(self.instance, name)
+
+    def __setattr__(self, name):
+        return setattr(self.instance, name)
+
+
+a = Singletone()
+a.val = 'first'
+print(a)
+
+b = Singletone()
+b.val = 'second'
+print(b)
+
+c = Singletone()
+c.val = 'third'
+
+print('a: ', a)
+print('b: ', b)
+print('c: ', c)

Python/3-dict.py

@@ -0,0 +1,26 @@
+class SingletoneDict:
+
+    _state_dict = {}
+
+    def __init__(self):
+        self.__dict__ = self._state_dict
+
+
+class Singletone(SingletoneDict):
+
+    def __init__(self, arg):
+        SingletoneDict.__init__(self)
+        self.val = arg
+
+    def __str__(self): 
+    	return self.val
+
+
+a = Singletone('first')
+print(a)
+b = Singletone('second')
+print(b)
+c = Singletone('third')
+print('a: ', a)
+print('b: ', b)
+print('c: ', c)

Python/4-class_variable.py

@@ -0,0 +1,19 @@
+class Singletone(object):
+
+    __instance = None
+
+    def __new__(cls, val):
+        if Singletone.__instance is None:
+            Singletone.__instance = object.__new__(cls)
+        Singletone.__instance.val = val
+        return Singletone.__instance
+
+
+a = Singletone('first')
+print(a)
+b = Singletone('second')
+print(b)
+c = Singletone('third')
+print('a: ', a)
+print('b: ', b)
+print('c: ', c)

Python/5-decorator.py

@@ -0,0 +1,30 @@
+class SingletoneDecorator:
+
+    def __init__(self, arg_class):
+        self.arg_class = arg_class
+        self.instance = None
+
+    def __call__(self, *args, **kwargs):
+        if self.instance == None:
+            self.instance = self.arg_class(*args, **kwargs)
+        return self.instance
+
+class MyClass: 
+	pass
+
+
+MyClass = SingletoneDecorator(MyClass)
+
+a = MyClass()
+b = MyClass()
+c = MyClass()
+
+a.val = 'first'
+b.val = 'second'
+c.val = 'third'
+
+print(a.val)
+print(b.val)
+print(c.val)
+
+print(a is b is c)

Python/6-metaclass.py

@@ -0,0 +1,35 @@
+class Metaclass(type):
+
+    def __init__(cls, name, bases, dict):
+
+        super(Metaclass, cls).__init__(name, bases, dict)
+
+        new_origin = cls.__new__
+
+        def new_custom(cls, *args, **kwargs):
+            if cls.instance == None:
+                cls.instance = new_origin(cls, *args, **kwargs)
+            return cls.instance
+
+        cls.instance = None
+        cls.__new__ = staticmethod(new_custom)
+
+class MyClass(object):
+
+    __metaclass__ = Metaclass
+
+    def __init__(self,val):
+        self.val = val
+
+    def __str__(self):
+        return repr(self) + self.val
+
+a = MyClass('first')
+b = MyClass('second')
+c = MyClass('third')
+
+print('a: ', a)
+print('b: ', b)
+print('c: ', c)
+
+print(a is b is c)