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• Official Website
• Installation
• Documentation

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Cometa.py is a lightweight, high-performance Python library binding for the libcardano-c library, designed to simplify blockchain development on Cardano.

Cometa.py packages libcardano-c using CFFI bindings, providing a fully documented, developer-friendly Pythonic API with type hints for excellent IDE support.

Example:

builder = TxBuilder(protocol_params, provider)

unsigned_tx = (
    builder
    .set_change_address(sender_address)
    .set_utxos(utxos)
    .send_lovelace(recipient_address, 12_000_000)
    .set_valid_until(current_time + 3600)
    .build()
)

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Conway Era Support

Cometa.py supports all features up to the Conway era, which is the current era of the Cardano blockchain. Conway era brought decentralized governance to Cardano, including:

• DRep Registration: Register as a Delegated Representative (DRep) using public keys or scripts
• Voting: Vote on governance proposals as a DRep, SPO, or Constitutional Committee member
• Governance Actions: Submit proposals for treasury withdrawals, parameter changes, hard forks, and more
• Stake Delegation: Delegate voting power to DReps

See the Documentation for more information on governance features.

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Installation

You can install Cometa.py using pip:

pip install biglup-cometa

Once installed, you can import it into your application:

import cometa

version = cometa.get_lib_version()
print(f"Library version: {version}")

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Getting Started

The primary component for creating transactions is the TxBuilder. It provides a fluent (chainable) API that simplifies the complex process of assembling inputs, outputs, and calculating fees.

First, establish a connection to the Cardano network using a Provider:

from cometa import BlockfrostProvider, NetworkMagic

provider = BlockfrostProvider(
    network=NetworkMagic.Preprod,
    project_id="YOUR_BLOCKFROST_PROJECT_ID"
)

Tip: You can create your own providers by implementing the Provider protocol.

Create your addresses and fetch UTxOs:

from cometa import Address

sender_address = Address.from_bech32("addr_test1...")
recipient_address = Address.from_bech32("addr_test1...")

# Fetch UTxOs from the provider
utxos = provider.get_utxos(sender_address)
protocol_params = provider.get_protocol_parameters()

Build your transaction using the fluent API:

from cometa import TxBuilder

builder = TxBuilder(protocol_params, provider)

unsigned_tx = (
    builder
    .set_change_address(sender_address)
    .set_utxos(utxos)
    .send_lovelace(recipient_address, 2_000_000)  # Send 2 ADA
    .set_valid_until(current_time + 7200)  # Set TTL
    .build()
)

Sign the transaction with your private key:

Submit the signed transaction:

tx_hash = provider.submit_transaction(signed_tx)
print(f"Transaction submitted! TxHash: {tx_hash}")

You can see the full capabilities of the transaction builder in the TxBuilder API documentation.

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Extending the Transaction Builder

The TxBuilder API allows you to override its core logic for coin selection and transaction evaluation. If these custom implementations are not provided, the builder uses the following defaults:

• Coin Selection: A "Largest First" strategy via LargeFirstCoinSelector
• Transaction Evaluation: A remote service via the configured Provider (e.g., Blockfrost)

Implementing a Custom CoinSelector

The coin selector is responsible for choosing which UTxOs to spend to cover the value required by the transaction's outputs. You can provide your own strategy by implementing the CoinSelector protocol:

from typing import List
from cometa import Utxo, TransactionOutput, Value

class MyCoinSelector:
    """Custom coin selection strategy."""

    @property
    def name(self) -> str:
        return "MyCustomSelector"

    def select(
        self,
        pre_selected_utxo: List[Utxo],
        available_utxo: List[Utxo],
        target: Value,
    ) -> Tuple[List[Utxo], List[Utxo]]:
        # Your custom selection logic here
        ...

Attach your custom selector to the builder:

my_selector = MyCoinSelector()
builder.set_coin_selector(my_selector)

Implementing a Custom TxEvaluator

The transaction evaluator is responsible for calculating the execution units (ExUnits) for any Plutus scripts in a transaction. You can provide a custom implementation:

from typing import List, Optional
from cometa import Transaction, Utxo, Redeemer

class MyTxEvaluator:
    """Custom transaction evaluator."""

    def get_name(self) -> str:
        return "MyCustomEvaluator"

    def evaluate(
        self,
        transaction: Transaction,
        additional_utxos: Optional[List[Utxo]] = None,
    ) -> List[Redeemer]:
        # Your custom evaluation logic here
        # Could use a local evaluator or different service
        ...

Attach your custom evaluator to the builder:

my_evaluator = MyTxEvaluator()
builder.set_evaluator(my_evaluator)

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Building and Testing

While the underlying libcardano-c library has its own comprehensive test suite, Cometa.py maintains a separate, dedicated suite of tests. These binding-level tests verify the correctness of the Python-to-C interface and ensure the high-level API functions as expected.

To build and run the tests, use the following commands:

pip install -e ".[dev]"
pytest

To run the linter:

pylint src/cometa

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License

Cometa.py is licensed under the Apache 2.0 License. See the LICENSE file for more information.