weight

Returns the weight of this transaction, as defined by BIP-141.

Transaction weight is defined as Base transaction size * 3 + Total transaction size (ie. the same method as calculating Block weight from Base size and Total size).

For transactions with an empty witness, this is simply the consensus-serialized size times four. For transactions with a witness, this is the non-witness consensus-serialized size multiplied by three plus the with-witness consensus-serialized size.

For transactions with no inputs, this function will return a value 2 less than the actual weight of the serialized transaction. The reason is that zero-input transactions, post-segwit, cannot be unambiguously serialized; we make a choice that adds two extra bytes. For more details see BIP 141 which uses a "input count" of 0x00 as a marker for a Segwit-encoded transaction.

If you need to use 0-input transactions, we strongly recommend you do so using the PSBT API. The unsigned transaction encoded within PSBT is always a non-segwit transaction and can therefore avoid this ambiguity.