Вложение классов

* В Python классы могут быть вложенными

# Singleton

class OnlyOne(object):
    class __OnlyOne:
        def __init__(self):
            self.val = None
        def __str__(self):
            return `self` + self.val
    instance = None
    def __new__(cls): # __new__ always a classmethod
        if not OnlyOne.instance:
            OnlyOne.instance = OnlyOne.__OnlyOne()
        return OnlyOne.instance
    def __getattr__(self, name):
        return getattr(self.instance, name)
    def __setattr__(self, name):
        return setattr(self.instance, name)