10_decode_tx.py

# SPDX-License-Identifier: MIT
"""10 - Decode a transaction and draw it: inputs on the left, outputs on the right.

Fetches a transaction from TestnetScan's public API (no node needed) and draws where
the coins came from and where they went, plus the fee. This is the read-only mirror of
recipe 08 (which builds one). On Bitcoin and Litecoin every amount and address is public,
which recipe 12 contrasts with Monero, where they are hidden.

    python 10_decode_tx.py <txid>

Get a txid from a recent block (recipe 11), the mempool (recipe 04), or the explorer.
"""
import sys
import requests
from config import api

if len(sys.argv) < 2:
    sys.exit("usage: python 10_decode_tx.py <txid>   (get one from 11_block.py or 04_mempool.py)")
txid = sys.argv[1]

tx = requests.get(api(f"/tx/{txid}"), timeout=30).json()
vsize = -(-tx["weight"] // 4)                       # ceil(weight / 4)
fee = tx.get("fee", 0)
rate = fee / vsize if vsize else 0
st = tx["status"]
where = f"confirmed at height {st['block_height']:,}" if st.get("confirmed") else "unconfirmed (in the mempool)"


def short(a):
    a = a or ""
    return a if len(a) <= 20 else f"{a[:11]}...{a[-6:]}"


def side(entry):                                    # "address              value"
    addr, val = entry
    amt = "" if val is None else format(val, ",")
    return f"{short(addr):<21}{amt:>11}"


ins = []
for v in tx["vin"]:
    if v.get("is_coinbase"):
        ins.append(("coinbase (new coins)", None))
    else:
        p = v.get("prevout") or {}
        ins.append((p.get("scriptpubkey_address") or f"[{p.get('scriptpubkey_type', '?')}]", p.get("value", 0)))
outs = [(o.get("scriptpubkey_address") or f"[{o.get('scriptpubkey_type', '?')}]", o.get("value", 0)) for o in tx["vout"]]
total_in = sum(v for _, v in ins if v is not None)
total_out = sum(v for _, v in outs)

print(f"tx {short(txid)}   {where}")
print(f"size {vsize:,} vB   fee {fee:,} sat   ({rate:.1f} sat/vB)")
print()
print(f"{'INPUTS (' + str(len(ins)) + ')':<32}    {'OUTPUTS (' + str(len(outs)) + ')'}")
for i in range(max(len(ins), len(outs))):
    left = side(ins[i]) if i < len(ins) else " " * 32
    arrow = " -> " if i == 0 else "    "
    right = side(outs[i]) if i < len(outs) else ""
    print(f"{left}{arrow}{right}".rstrip())
print()
if total_in:
    print(f"in {total_in:,} sat  ->  out {total_out:,} sat  +  fee {fee:,} sat")
else:
    print(f"out {total_out:,} sat   (coinbase: inputs are newly issued coins plus fees)")