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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 _encode(self):
 """Encode the message and return a bytestring."""

        data = ByteBuffer()
        if not hasattr(self, '__fields__'):
            return data.tostring()

        for field in self.__fields__:
            field.encode(self, data)
        return data.tostring()