Gets the block CONFIRMATION_BLOCKS in the past and returns its block hash

def get_confirmed_blockhash(self):
        """ Gets the block CONFIRMATION_BLOCKS in the past and returns its block hash """
        confirmed_block_number = self.web3.eth.blockNumber - self.default_block_num_confirmations
        if confirmed_block_number < 0:
            confirmed_block_number = 0

        return self.blockhash_from_blocknumber(confirmed_block_number)