Python SDK - Binary Canonical Serialization (BCS) Format

Transaction arguments for the Aptos Python SDK are encoded with Binary Canonical Serialization (BCS). This is the format the Aptos chain recognizes, with specific types such as u64 or u128 instead of a generic integer.

Wrap each argument in TransactionArgument with the matching serializer:

from aptos_sdk.v2.bcs import Serializer
from aptos_sdk.v2.transactions import EntryFunction, TransactionArgument, TransactionPayload
from aptos_sdk.v2.types import StructTag, TypeTag

payload = EntryFunction.natural(
    "0x1::aptos_account",
    "transfer_coins",
    [TypeTag(StructTag.from_str("0x1::aptos_coin::AptosCoin"))],
    [
        TransactionArgument(bob.address, Serializer.struct),
        TransactionArgument(1_000, Serializer.u64),
    ],
)
raw_txn = await aptos.transaction.build(
    sender=alice.address,
    payload=TransactionPayload(payload),
)

Common serializer methods include Serializer.struct, Serializer.u8, Serializer.u16, Serializer.u32, Serializer.u64, Serializer.u128, Serializer.u256, Serializer.bool, Serializer.str, and Serializer.to_bytes. Signed integers (i8 through i256) are also supported.

You can also serialize values directly:

from aptos_sdk.v2.bcs import Serializer, Deserializer

ser = Serializer()
ser.u64(1_000)
ser.bool(True)
data = ser.output()

deser = Deserializer(data)
assert deser.u64() == 1_000
assert deser.bool() is True

You can learn more about BCS by exploring the BCS GitHub repo.