Get the serialized value for a given key.

def convert(self, key, value):
        """Get the serialized value for a given key."""
        if key not in self._dtypes:
            self.read_types()
            if key not in self._dtypes:
                name = utils.name(value)
                serializer = utils.serializer(name)
                deserializer = utils.deserializer(name)
                self._dtypes[key] = (name, serializer, deserializer)
                with self.db:
                    self.db.execute("replace into value_types (key, value_type) values (?, ?)", (key, name))
        return self._dtypes[key][1](value)