class Field:
    def __set_name__(self, owner, name):
        self.name = name

    def __get__(self, inst, cls):
        if inst is None:
            return self
        assert self.name is not None
        return inst._fields[self.name]

    def __set__(self, inst, value):
        assert self.name is not None
        inst._fields[self.name] = value

    def _set_namespace(self, inst, name, namespaces):
        if name is None:
            return

        if name not in namespaces:
            raise KeyError("namespace %s not found" % (name))
        inst._namespace_prefix = name

    def _call(self, inst, method, *args):
        call = getattr(inst, method or '', None)

        if callable(call):
            return call(*args)