Cardano transactions are usually involved with three child components, transaction input(s), transaction output(s), and transaction fee. There are two approaches of creating transactions in PyCardano. The first one is to provide child components explicitly, which is also referred as creating “raw transactions”. The second one is to use a transaction builder, which is usually more user-friendly.
Below are two examples that generates the same transaction using different approaches. The transaction is simply sending 100000 ADA to ourselves, and paying the network fees.
Raw transactions can be precisely constructed by specifying inputs, outputs, fees, etc. This approach is usually for power users or for debugging purposes. For most users, a transaction builder will be the way to go. If you are interested in learning more about building raw transactions, keep reading. Otherwise, please skip this section and directly jump to Transaction builder section.
Define Tx input:
>>> from pycardano import ( ... PaymentSigningKey, ... PaymentVerificationKey, ... Transaction, ... TransactionBody, ... TransactionInput, ... TransactionOutput, ... TransactionWitnessSet, ... VerificationKeyWitness, ... ) >>> # Assume the UTxO is sitting at index 0 of tx 732bfd67e66be8e8288349fcaaa2294973ef6271cc189a239bb431275401b8e5 >>> tx_id = "732bfd67e66be8e8288349fcaaa2294973ef6271cc189a239bb431275401b8e5" >>> tx_in = TransactionInput.from_primitive([tx_id, 0])
Define Tx output. Suppose we have total of 900000000000 lovelace, and we need to pay 165897 as network fee, then we will get 799999834103 as change:
>>> address = "addr_test1vrm9x2zsux7va6w892g38tvchnzahvcd9tykqf3ygnmwtaqyfg52x" >>> # Define two transaction outputs, the first one is the amount we want to send, the second one is the change. >>> output1 = TransactionOutput.from_primitive([address, 100000000000]) >>> output2 = TransactionOutput.from_primitive([address, 799999834103])
Create a transaction body from the input and outputs defined above and add transaction fee:
>>> tx_body = TransactionBody(inputs=[tx_in], outputs=[output1, output2], fee=165897)
Transaction ID could be obtained when we are done configuring the transaction body, because is essentially the hash of TransactionBody:
>>> tx_body.id TransactionId(hex='1d40b950ded3a144fb4c100d1cf8b85719da91b06845530e34a0304427692ce4')
Sign the transaction body hash and create a complete transaction:
>>> sk = PaymentSigningKey.load("path/to/payment.skey") >>> # Derive a verification key from the signing key >>> vk = PaymentVerificationKey.from_signing_key(sk) >>> # Sign the transaction body hash >>> signature = sk.sign(tx_body.hash()) >>> # Alternatively, we can sign the transaction ID as well >>> signature_alternative = sk.sign(tx_body.id.payload) >>> assert signature == signature_alternative >>> # Add verification key and the signature to the witness set >>> vk_witnesses = [VerificationKeyWitness(vk, signature)] >>> # Create final signed transaction >>> signed_tx = Transaction(tx_body, TransactionWitnessSet(vkey_witnesses=vk_witnesses))
A complete example could be found here.
Notice that, to create a transaction, we need to know which transaction input to use, the amount of changes to return to the sender, and the amount of fee to pay to the network, which is possible to calculate, but requiring additional efforts. Instead, we can use a transaction builder to help us autofill in these information.
To use a transaction builder, we first need to create a chain context, so the builder can read protocol parameters and search proper transaction inputs to use. Currently, the available chain context is BlockFrostChainContext
>>> from blockfrost import ApiUrls >>> from pycardano import BlockFrostChainContext >>> network = Network.TESTNET >>> context = BlockFrostChainContext("your_blockfrost_project_id", base_url=ApiUrls.preprod.value)
Read signing key into the program and generate its corresponding verification key:
>>> from pycardano import PaymentSigningKey, PaymentVerificationKey, Address, Network >>> network = Network.TESTNET >>> sk = PaymentSigningKey.load("path/to/payment.skey") >>> vk = PaymentVerificationKey.from_signing_key(sk) >>> address = Address(pvk.hash(), svk.hash(), network)
Create a transaction builder from chain context:
>>> builder = TransactionBuilder(context)
Tell the builder that transaction input will come from our own address:
Specify output amount:
>>> builder.add_output(TransactionOutput.from_primitive([address, 100000000000]))
Add additional transaction information as needed:
>>> builder.ttl = 3600 >>> builder.reference_inputs.add(tx_in)
Create a signed transaction using transaction builder. Unlike building a raw transaction, where we need to manually sign a transaction and build a transaction witness set, transaction builder can build and sign a transaction directly with its build_and_sign method. The code below tells the builder to build a transaction and sign the transaction with a list of signing keys (in this case, we only need the signature from one signing key, sk) and send the change back to sender’s address:
>>> signed_tx = builder.build_and_sign([sk], change_address=address)
Transaction ID could be obtained from the transaction object:
>>> tx.id TransactionId(hex='1d40b950ded3a144fb4c100d1cf8b85719da91b06845530e34a0304427692ce4')
By using transaction builder, we no longer need to specify which UTxO to use as transaction input or calculate transaction fee, because they are taken care by the transaction builder. Also, the code becomes much more concise.
A more complex example of using transaction builder could be found in this Github example.
Once we have a signed transaction, it could be submitted to the network. The easiest way to do so is through a chain context: