Retrieve the value returned by a function which was invoked with 'transact'

Question

I am using Python web3 in order to deploy and test a Solidity contract.

Here are my initial steps:

from web3 import Web3
from web3 import RPCProvider
from json import loads

eth = Web3(RPCProvider()).eth
abi = open(ABI_FILE_NAME).read()
bin = open(BIN_FILE_NAME).read()
transaction = {'from':eth.accounts[0]}

contract = eth.contract(abi=loads(abi),bytecode=bin)
tx_hash  = contract.deploy(transaction=transaction)
address  = eth.getTransactionReceipt(tx_hash)['contractAddress']
handle   = contract(address).transact(transaction)

At this point, I invoke a function in the contract:

tx_hash = handle.some_func()

And this is what I get when I call eth.getTransactionReceipt(tx_hash):

{
    'transactionHash'   : '0x...',
    'transactionIndex'  : 0,
    'blockHash'         : '0x...',
    'blockNumber'       : 3710,
    'gasUsed'           : 72597,
    'cumulativeGasUsed' : 72597,
    'contractAddress'   : None,
    'logs'              : [],
    'status'            : 1
}

How can I retrieve the actual value that some_func has returned?

I found a very detailed answer which seems to refer to this issue, but I have not been able to conclude the actual steps required.

Thank you.

Answer 1

Unfortunately, there is no "return value" for a transaction. There are two typical patterns to simulate a return value:

1. Mutate the state via a transaction and then read the resulting state by calling a view function. (Note that this gives you the state at the time you call the view function, not necessarily the state right after the transaction, since other transactions may have come between.)
2. Mutate the state via a transaction and emit an event that's observed by the caller.

Depending on your scenario, one or the other may be a more natural fit.

Answer 2

Here is how I was able to resolve this problem:

1. In the Solidity contract, just before returning, I generate an event which contains the value about to be returned.

2. In the Python code, I extract the returned value with:

   retVal = eth.getTransactionReceipt(tx_hash)['logs'][0]['data']

This value happens to be an hexadecimal string in my case, so I use int(retVal,16) in order to obtain the corresponding integer value.