How to compute a TXID of any bitcoin transaction in Python

Question

there, I've already looked at other similar posts on here but with little luck. All im trying to do is calculate the TXID of a transaction after downloading the transaction in hexadecimal from a blockchain explorer VIA an API.

The problem is that the TXID calculated Isn't the TXID we expect.

Here's the code, I'm using bitcoin-utils

from bitcoinutils.transactions import Transaction
import requests


# Fetch the raw transaction data from the blockchain.info API
tx_hash = '9d2d91b764ba2559b098cdd30525e2160bb15c6e27d6869e7b121a9037d8bd86'
api_response = requests.get(f'https://blockchain.info/rawtx/{tx_hash}?format=hex')

if api_response.status_code == 200:
    raw_tx_hex = str(api_response.text)
else:
    print(f"Error fetching transaction data: {api_response.status_code}")


mytrans = Transaction.from_raw(raw_tx_hex)
print(mytrans.get_txid())

Answer 1

That's a SegWit transaction, not all of it is part of the transaction hash preimage (see here for details). You have to exclude the SegWit parts. This Bash one-liner returns the correct TXID:

echo '0100000001189a3f6fbfb614264c8176cd2d3836882afc3c0940d698d6b481f3e5cb733c200000000000ffffffff0240420f00000000001976a914341b568f59229818c460b1795ad48cd78895c54d88ac6eeefa4a00000000160014d701ce5e753bd9454d343c8a3b86d84a3c34dbf500000000' | tr -d '\n' | xxd -r -p | sha256sum | grep -oE '^[0-9a-f]+' | xxd -r -p | sha256sum | grep -oE '^[0-9a-f]+' | tac -rs '..'; echo

Output

9d2d91b764ba2559b098cdd30525e2160bb15c6e27d6869e7b121a9037d8bd86

Explanation

Your TX can be disassembled as follows, and you have to hash everything except the bold parts:

TX version: 01000000
Witness marker and flag: 0001
Input count: 01
Prevout reference: 189a3f6fbfb614264c8176cd2d3836882afc3c0940d698d6b481f3e5cb733c2000000000
Unlocking script length: 00
Unlocking script: EMPTY (note: non-SegWit inputs will have script length >0 and have the unlocking script here instead of in the witness, so for non-SegWit this will not be EMPTY)
Sequence: ffffffff
Output count: 02
Output 01: 40420f00000000001976a914341b568f59229818c460b1795ad48cd78895c54d88ac
Output 02: 6eeefa4a00000000160014d701ce5e753bd9454d343c8a3b86d84a3c34dbf5
Witness count: 02
Witness 01: 473044022001609cd43eb8e9b8f8438eded9f6b10bad32efd7620724ccd2ed5277c0c6a3ae02200f0c1c3f4c409ada536d2363a2d8bdad418df67fed9b36bfa4482bd9985bf39601
Witness 02: 2102ee3c98964dd1bfe13bee16c0b95fcf8281f12c5885d1fcb7b59fc2cb01ca7632
TX locktime: 00000000

Concatenating only the non-SegWit parts gives us the TX hash preimage: 0100000001189a3f6fbfb614264c8176cd2d3836882afc3c0940d698d6b481f3e5cb733c200000000000ffffffff0240420f00000000001976a914341b568f59229818c460b1795ad48cd78895c54d88ac6eeefa4a00000000160014d701ce5e753bd9454d343c8a3b86d84a3c34dbf500000000