Gets all instances implementing type <_type>

def _get_all(self, _type):
        """ Gets all instances implementing type <_type> """
        if _type not in self.modules:
            raise ValueError("No such module, %s" % _type)
        if not self.insts_implementing.get(_type, None):
            raise ValueError("No instance implementing %s" % _type)
        return self.insts_implementing[_type]