====== Python: Singleton ======

<code python>
# class Singleton(type):
#     _instances = {}
#     def __call__(cls, *args, **kwargs):
#         if cls not in cls._instances:
#             cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
#         return cls._instances[cls]
              
class Singleton(type):
     _instances = {}
     def __call__(cls, *args, **kwargs):
         cls._instances.setdefault(cls, super().__call__(*args, **kwargs))
         return cls._instances[cls]

      
# Python2
class MyClass(BaseClass):
    __metaclass__ = Singleton
    pass
  

# Python3
class MyClass(BaseClass, metaclass=Singleton):
    pass
  

# Test
for x in range(5):
    MyClass().x = x
    print(MyClass().x)
</code>

  * https://medium.com/techtofreedom/3-levels-of-understanding-the-singleton-pattern-in-python-4bf429a10438  
  * http://stackoverflow.com/questions/6760685/creating-a-singleton-in-python