py-evm output with extra bytes

Question

I have simple smart contract

pragma solidity ^0.4.17;

contract Inbox {
    string public message;

    constructor()  public payable {
        message = "Hi there!";
    }

    function setMessage(string newMessage) public {
       message = newMessage;
    }
}

I've deployed it to my local py-evm instance. I tried to execute message() method. I com do following:

vm = chain.get_vm()                                                     

nonce = vm.state.account_db.get_nonce(SENDER)                           

vm_tx = vm.create_unsigned_transaction(                                 
         nonce=nonce,                                                        
         gas_price=gasprice,                                                 
         gas=startgas,                                                       
         to=decode_hex('febb9c06ccc7d378059c03f0c50f848c39d96fb6'),  
         value=value,                                                        
         data=payload,                                                       
     )                                                                       
signed_tx = vm_tx.as_signed_transaction(SENDER_PRIVATE_KEY)             
new_block, receipt, computation = chain.apply_transaction(signed_tx)

and in computation.output I got: b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00 \x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\tHi there!\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'

I can see "Hi there!" but why there are extra bytes? How to get clean output?

Answer 1

In general, you'll probably want to interact with a node by using a Web3.py Contract to handle all the ABI type conversions. But since you're using py-evm directly... (and trinity won't support eth_call until the next release in a couple weeks):

You can use eth-abi v1.1.1 to decode the data:

from eth_abi import decode_abi

decoded_result = decode_abi(['string'], computation.output)

assert decoded_result == (b'Hi there!',)

Note that the next major version of eth-abi will also decode the bytes to a str, using utf-8. Until then, you can:

decoded_str = decoded_result[0].decode()

assert decoded_str == 'Hi there!'