Python-сообщество

from StringIO import StringIO
from unittest import TestCase
class MetaChained(type):
    _instances = {}
    def __new__(cls, name, bases, dct):
        new_cls = super(MetaChained, cls).__new__(cls, name, bases, dct)
        cls._instances[name] = new_cls
        return new_cls
class Chained(object):
    __metaclass__ = MetaChained
    value = 'Chained'
    def __init__(self):
        self._chained = None
    def _add_in_chain(self, next):
        if self._chained:
            self._chained._add_in_chain(next)
        else:
            self._chained = next
    def __call__(self, out):
        out.write(self.value)
        if self._chained:
            self._chained(out)
    def __getattribute__(self, item):
        try:
            return super(Chained, self).__getattribute__(item)
        except AttributeError, e:
            dct = self.__metaclass__._instances
            if item in dct:
                def constructor(*args, **kwargs):
                    obj = dct[item](*args, **kwargs)
                    self._add_in_chain(obj)
                    return self
                return constructor
            else:
                raise e
class chainedA(Chained):
    value = 'A'
class chainedB(Chained):
    value = 'B'
class chainedC(Chained):
    value = 'C'
class TestChained(TestCase):
    def test_chained(self):
        out = StringIO()
        chain = chainedA().chainedB().chainedC()
        chain(out)
        self.assertEqual('ABC', out.getvalue())

chain = chainedA().chainedB().chainedC()

chain = chainedA(chainedB(chainedC()))