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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def handle_req(msg):
 """Process a req message. This will return the value if it exists. If no value exists, nothing is returned. """

    if not msg.gateway.is_sensor(msg.node_id, msg.child_id):
        return None
    value = msg.gateway.sensors[msg.node_id].children[
        msg.child_id].values.get(msg.sub_type)
    if value is not None:
        return msg.copy(
            type=msg.gateway.const.MessageType.set, payload=value)
    return None