class SedlacekStorage(object):
    """
    Please note, that every SedlacekStorage subclass is a singleton. You can't
    use sync & async versions of +SedlacekStorage in the same process.
    """
    _tornado_client = None
    _asyncio_client = None

    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            # protect parent from our specific args
            kwargscopy = dict(kwargs)
            for key in ['async', 'timeout']:
                if key in kwargscopy:
                    del kwargscopy[key]
            cls._instance = super(SedlacekStorage, cls) \
                .__new__(cls, *args, **kwargscopy)
        return cls._instance


class A(SedlacekStorage):
    _instance = None


class B(A):
    pass


a1 = A()
print(A._instance, B._instance)

b1 = B()
b2 = B()

print(b1 is b2)
print(a1 is b2)