how to send transaction with web3.py

Question

I'm trying to use web3.py with infura.io to interact with my smartcontract. here is my simple greeter contract code

pragma solidity ^0.5.7;
contract greeter{
    string greeting;

    function greet(string memory _greeting)public {
        greeting=_greeting;
    }
    function getGreeting() public view returns(string memory) {
        return greeting;
    }
}

I deployed it on ropsten testnet. with web3.py I can read greeting from chain. I wanted to add new greeting string with greet function. but somehow I have no idea about signing transaction. could anyone help me in understanding how to sign transacion and adding greeting to the contract using web3.py?

Answer 1

In web3py you can use the method buildTransaction:

myContract.functions.myMethod(*args, **kwargs).buildTransaction(transaction)

In your example you can do as follows:

tx = greeter.functions.greet("newGreet").buildTransaction({'nonce': web3.eth.getTransactionCount('your accountaddress')}}

Then you can sign the transaction using:

signed_tx = web3.eth.account.signTransaction(tx, private_key='your privateKey')

and finally you can send the transaction signed using:

web3.eth.sendRawTransaction(signed_tx.rawTransaction)

Hope this helps.