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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 build_block(self):
 """ Assembles the candidate block into it's finalized form for broadcast. """

        header_bytes = self.block_header.SerializeToString()
        block = Block(header=header_bytes,
                      header_signature=self._header_signature)
        block.batches.extend(self.batches)
        return block