I'm using Web3.py, and I'm trying to build a transaction to call a contract function where one of the arguments is of type "bytes". I want to call the function with a zero-length byte array.

However, when I call buildTransaction(), the zero-length byte array is encoded as a sequence of 128 zeros. I instead want a sequence of 64 zeros (representing the 32-byte length of the zero-length array, as per the Ethereum encoding specs).

I have tried using "0x", HexBytes('0x'), HexBytes(''), '', and b'' as the parameter, without any success.

Does anyone know how to accomplish what I need?

Edit: Here is a simple example to showcase what I mean.


    "constant": false,
    "inputs": [
        "name": "test",
        "type": "bytes"
    "name": "function",
    "outputs": [],
    "payable": true,
    "stateMutability": "payable",
    "type": "function"



import json, web3

dummy_address = '0xcbDc0bca6706da81d66e9928d3575dAd4F1a929b'

with open('/home/ubuntu/test.json', 'r') as file:
    abi = json.load(file)

w3 = web3.Web3(web3.Web3.HTTPProvider('https://mainnet.infura.io/v3/my_key'))

contract = w3.eth.contract(dummy_address, abi = abi)

params = { 'chainId': 1, 'type': 2, 'value': 0, 'from': dummy_address, 'gas': 1, 'maxFeePerGas': 1, 'maxPriorityFeePerGas': 1, 'nonce': 0 }

txn = contract.functions.function('0x').buildTransaction(params)



"4f626cf1" is the function selector, "0000000000000000000000000000000000000000000000000000000000000020" is the data location, and that is followed by 128 zeros (instead of the desired 64 zeros)

1 Answer 1


Well I ended up figuring this out myself I think. In case anyone else runs into this issue, here is the relevant code at site-packages/eth_abi/encoding.py:

    def encode(cls, value):

        if not value:
            padded_value = b'\x00' * 32
            padded_value = zpad_right(value, ceil32(len(value)))

        encoded_size = encode_uint_256(len(value))
        encoded_value = encoded_size + padded_value

        return encoded_value

That 5th line can be changed to

padded_value = b''

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