Tx build fails when specifying utxo to be consumed (object has no attribute 'output')

[ajuggler]

In this example I try to build a transaction where the UTxO to be consumed has been explicitly specified. It doesn't work, and throws the error

    selected_amount += i.output.amount
                       ^^^^^^^^
    AttributeError: 'TransactionInput' object has no attribute 'output'

which I interpret as PyCardano being unable to query the blockchain for the output amount associated with the provided UTxO. Have I found a bug in PyCardano? Here is the code:

import os
from pycardano import (
    Transaction,
    TransactionBody,
    TransactionInput,
    TransactionOutput,
    TransactionBuilder,
    BlockFrostChainContext,
    Value,
    Network,
    Address,
)
from blockfrost import ApiUrls

context = BlockFrostChainContext("my-consealed-preview-project-id", base_url=ApiUrls.preview.value)
NETWORK = Network.TESTNET

# ADDRESSES

wallet2_address = 'addr_test1vznjse8fycd7dzw9usre7vka0rwjcqszuu7a26pkdrw0g8sczt0l5'

# INPUTS

tx_in = TransactionInput.from_primitive(
    ['4a16c4f23a1855392496650deac62eb6c1fa1887f7a2e59ee2a3a3c7b67fa733', 1])

# OUTPUTS

tx_out = TransactionOutput(address=wallet2_address, amount=Value(5000000))

# BUILD TRANSACTION                                                          

tx_builder = TransactionBuilder(context)

tx_builder.add_input(tx_in);

tx_builder.add_output(tx_out);

tx_builder.build()

[nielstron]

Please try i.input.amount instead. The error displayed is an attribute error and is related to using the wrong object to achieve what you want to do, not a bug in PyCardano.

[cffls]

tx_in defined in your script should be a UTxO type instead of a TransactionInput.

[ajuggler]

As cffls' answer states, tx_in should be a UTxO, which is an object with input and output attributes. With this in mind, I figured out the correct code:

# Recall that my objective is to consume a specific UTxO, now determined by variable 'tx_in_input'

# ADDRESSES

wallet1_address = 'addr_test1vq8y2nlht8ek0elatvksa5ctg6u4eatrj6hhqz3ln9ca0gsn3cnam'

wallet2_address = 'addr_test1vznjse8fycd7dzw9usre7vka0rwjcqszuu7a26pkdrw0g8sczt0l5'

# INPUTS

tx_in_input = TransactionInput.from_primitive(
    ['4a16c4f23a1855392496650deac62eb6c1fa1887f7a2e59ee2a3a3c7b67fa733', 1])

tx_in = next(
    (
        u for u in context.utxos(wallet1_address)
        if u.input == tx_in_input
    ),
    None
)

# OUTPUTS

tx_out = TransactionOutput(address=wallet2_address, amount=Value(5000000))

# BUILD TRANSACTION                                                          

tx_builder = TransactionBuilder(context)

tx_builder.add_input(tx_in);

tx_builder.add_output(tx_out);

tx_builder.build()

The only drawback is that it is not enough to know tx_in_input, as also the wallet address where it is locked (wallet1 in this case) is also needed. I think there is no way around this.

[cffls]

You can simplify the script by providing wallet1_address to method add_input_address in txbuilder, something like this:

tx_builder.add_input_address(wallet1_address)
tx_builder.add_output(tx_out);
tx_builder.build()

Then the tx builder will search for all utxos under this wallet address when building the transaction.

[ajuggler]

But I don't think this would guarantee that the transaction consumes the particular UTxO that I want it to consume. (Why would I insist in picking a particular UTxO? Maybe because this is part of a minting transaction and the minting policy enforces consumption of a particular UTxO in order to guarantee that an NFT is in fact unique.)

[cffls]

Okay, in that case you can list utxos using context and loop through all utxos until you see the one you want to spend:

utxo_in = None
tx_in_input = TransactionInput.from_primitive(
    ['4a16c4f23a1855392496650deac62eb6c1fa1887f7a2e59ee2a3a3c7b67fa733', 1])
utxos = context.utxos(wallet1_address)
for utxo in utxos:
    if utxo.intput == tx_in_input
    utxo_in = utxo

tx_builder.add_input(utxo_in)
tx_builder.add_output(tx_out);
tx_builder.build()