cardano-checkout-py/README.md

# cardano-checkout

Merchant-side Cardano payment lifecycle in Python. Zero-custody.

Ships the invoice state machine + UTxO watcher + reprice loop.
Per-invoice HD-derived receive addresses, Koios polling, confirm /
underpay / overpay classification, time-windowed repricing against
your own oracle.

Does NOT ship Cardano primitives. Address derivation, chain context,
transaction building, native-script minting, signing — use
[pycardano](https://github.com/Python-Cardano/pycardano) directly.
This library slots next to it.

## Quick start

```python
import asyncio
from datetime import datetime, timedelta, timezone

from cardano_checkout import (
    Invoice, InvoiceStatus, InMemoryStore, InvoiceScheduler,
)


# Your oracle. Anything async returning int lovelace works.
async def my_price_fn(usd: float) -> int:
    rate = await fetch_ada_usd_somewhere()   # CoinGecko, Koios, fixed rate, etc.
    return int(round(usd / rate * 1_000_000))


async def main() -> None:
    store = InMemoryStore()  # swap for your SQLAlchemy / asyncpg / sqlite adapter

    invoice = Invoice(
        id="ord-0042",
        merchant_id="my-shop",
        derivation_index=42,
        receive_address="addr1q...",  # derive via pycardano
        expected_lovelace=5_000_000,
        usd_amount=2.50,
        expires_at=datetime.now(timezone.utc) + timedelta(minutes=15),
    )
    await store.create(invoice)

    scheduler = InvoiceScheduler(store=store, price_fn=my_price_fn)
    await scheduler.start()

    # ... app runs ...

    await scheduler.stop()

asyncio.run(main())
```

## Deriving addresses with pycardano

```python
from pycardano import HDWallet, Address, Network

# Account-level xpub — public, not a secret.
xpub_hex = "..."

account = HDWallet.from_xpub(bytes.fromhex(xpub_hex))

def derive_address(account: HDWallet, index: int, network=Network.MAINNET) -> str:
    payment = account.derive(0).derive(index)   # external chain, address index
    staking = account.derive(2).derive(0)       # staking chain, always index 0
    addr = Address(
        payment_part=payment.public_key.hash(),
        staking_part=staking.public_key.hash(),
        network=network,
    )
    return str(addr)

addr = derive_address(account, index=42)
```

## NFT cert: CIP-25 v2 metadata

Copy-paste builder for an on-chain cert per paid order. No dep.

```python
def build_cip25_metadata(
    *,
    policy_id: str,
    asset_name: str,
    name: str,
    image_cid: str,
    description: str = "",
    media_type: str = "image/jpeg",
    properties: dict | None = None,
) -> dict:
    """Build a CIP-25 v2 metadata envelope.

    Returns a dict ready to submit as transaction metadatum label 721.
    Handles the 64-char chunking rule for long descriptions.
    """
    def chunk64(s: str) -> list[str]:
        if len(s) <= 64:
            return [s]
        return [s[i:i + 64] for i in range(0, len(s), 64)]

    body: dict = {
        "name": name,
        "image": f"ipfs://{image_cid}",
        "mediaType": media_type,
    }
    if description:
        body["description"] = description if len(description) <= 64 else chunk64(description)
    if properties:
        body.update(properties)

    return {
        "721": {
            policy_id: {asset_name: body},
            "version": "2.0",
        }
    }
```

Hand the dict to pycardano's `AuxiliaryData(Metadata({...}))` when
building the mint tx.

## Implementing your own InvoiceStore

`InvoiceStore` is a Protocol — implement six methods against whatever
backend you want (SQLAlchemy, asyncpg, SQLite, in-memory).

```python
from cardano_checkout import Invoice, InvoiceStatus, InvoiceStore

class MySqliteStore:
    async def create(self, invoice: Invoice) -> None: ...
    async def get(self, invoice_id: str) -> Invoice | None: ...
    async def list_by_status(self, status: InvoiceStatus, limit: int = 100) -> list[Invoice]: ...
    async def update(self, invoice: Invoice) -> None: ...
    async def next_derivation_index(self, merchant_id: str) -> int: ...
    async def record_tx(self, invoice_id: str, tx_hash: str, lovelace_delta: int) -> None: ...
```

See `InMemoryStore` in `cardano_checkout/store.py` for a reference impl.

## Modules

| Module | Purpose |
|---|---|
| `invoice.py` | `Invoice` dataclass + `InvoiceStatus` enum |
| `store.py`   | `InvoiceStore` Protocol + `InMemoryStore` reference impl |
| `monitor.py` | `check_address_utxos` (Koios), `evaluate_utxos`, `check_pending_invoices`, `reprice_expired_invoices` |
| `scheduler.py` | `InvoiceScheduler` — APScheduler wrapper, 15s check + 60s reprice |

Two direct deps: `httpx`, `apscheduler`. No pycardano dep.

## Design

1. **Protocol-first.** Persistence, pricing, side-effects through
   consumer-supplied interfaces.
2. **Use pycardano directly.** No wrapping of primitives.
3. **Zero-custody.** Merchant keys never touch this code. xpub-derived
   addresses, UTxO observation, state transitions. Funds flow directly
   between customer and merchant wallets.
4. **Offline-first tests.** Koios + price oracles stubbed via fixture.

## License

Apache-2.0.