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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 get(self, block_number: BlockNumber) -> str: """Given a block number returns the hex representation of the blockhash"""

        if block_number in self.mapping:
            return self.mapping[block_number]

        block_hash = self.web3.eth.getBlock(block_number)['hash']
        block_hash = block_hash.hex()
        self.mapping[block_number] = block_hash
        return block_hash