Call function from contract using sendTransaction() - web3.py

Question

I have contract Test.sol migrated to privatenet running on port '8545' which updates value of 'a'.

Test.sol

pragma solidity ^0.4.20;

contract Test{

 uint public a=2;

function updateval(uint b) public {

    a = a+ b;
}

function geta() public view returns(uint c){
    c=a;
    return c;
}

}

I am using python3.6 to call function updateval() from Test.sol.

contractcall.py

import json
import web3
import binascii
from web3 import Web3,HTTPProvider
from solc import compile_source
from web3.contract import ConciseContract 

w3 = Web3(HTTPProvider('http://127.0.0.1:8545'))


abi= '''[
{
    "constant": false,
    "inputs": [
        {
            "name": "b",
            "type": "uint256"
        }
    ],
    "name": "updateval",
    "outputs": [],
    "payable": false,
    "stateMutability": "nonpayable",
    "type": "function"
},
{
    "constant": true,
    "inputs": [],
    "name": "a",
    "outputs": [
        {
            "name": "",
            "type": "uint256"
        }
    ],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
},
{
    "constant": true,
    "inputs": [],
    "name": "geta",
    "outputs": [
        {
            "name": "c",
            "type": "uint256"
        }
    ],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
}
]
'''

address = Web3.toChecksumAddress("0x09f2e386111647ff986439f488583f39e153c940") //ContractAddress
mywalletaddress = w3.eth.accounts[0]

Test = w3.eth.contract(address, abi=abi)

b=4
msg = w3.toHex(text='Hello World')

txnHash= Test.functions.updateval(int(b)).sendTransaction({'from' : mywalletaddress,'data' : msg })
print(txnHash)

I need to pass message 'Hello world' with transaction. I tried with the following code:

txnHash = Test.functions.updateval(b).buildTransaction({'from':mywalletaddress})

print(txnHash)

tx= w3.eth.sendTransaction(txnHash)//How to pass data field here??
print('tx',tx)

What is the correct method of implementing it. Help will be appreciated.

Answer 1

txnHash = Test.functions.updateval(int(b)).sendTransaction(
  {'from' : mywalletaddress, 'data' : msg }
)

There are a few problems with this:

1. sendTransaction isn't one of the ContractFunction methods. You're probably looking for transact()
2. The data field of a transaction is used to invoke the function on the smart contract. There is no good reason to override it when building the function this way. Overriding will cause the contract function invocation to fail.

It's not clear why you:

need to pass message 'Hello world' with transaction.

You can't do that with the current contract. Perhaps you could add a string argument to the function you are calling, and then pass the string in, like:

function updateval(uint b, string message) public {
    a = a + b;
    // do something with message
}

Then you could send the transaction with:

txnHash = Test.functions.updateval(b, msg).sendTransaction(
  {'from' : mywalletaddress}
)