'Only sign the block if we have a (completed) transaction in the market community with the specific txid.'

def should_sign(self, message):

    if (not self.market_community):
        return False
    (trader_id_str, transaction_number_str) = message.payload.block.transaction['txid'].split('.')
    txid = TransactionId(TraderId(trader_id_str), TransactionNumber(int(transaction_number_str)))
    transaction = self.market_community.transaction_manager.find_by_id(txid)
    return bool(transaction)
