class ClassRegistry(object):

    def __init__(self, name):
        self.name = name
        self._classes = {}
        self._callbacks = {}

    def _resolve_name(self, classname, relative_to):
        parts = classname.split('.')
        classname = parts[-1]
        module = '.'.join(parts[:-1])
        if not module:
            module = relative_to
        return module, classname

    def register(self, cls):
        module, classname = (cls.__module__, cls.__name__)
        self._classes[(module, classname)] = cls
        if (module, classname) in self._callbacks:
            for func in self._callbacks[(module, classname)]:
                func(cls)
            del self._callbacks[(module, classname)]

    def get_class(self, classname, relative_to=None):
        module, classname = self._resolve_name(classname, relative_to)
        return self._classes[(module, classname)]

    def register_callback(self, callback, classname, relative_to=None):
        module, classname = self._resolve_name(classname, relative_to)
        if (module, classname) in self._classes:
            callback(self._classes[(module, classname)])
        else:
            self._callbacks.setdefault((module, classname), []).append(
                callback)