Where is the result of the smart contract call stored when using web3.py's transact method?

Question

I am using web3.py and trying to get the return value of a smart contract method after calling it. Of course, I can do it like this:

value = smart_contract_instance.someFunction()

but this does not give me any information about how much gas was used. If I use the transact method like this:

tx_hash = smart_contract_instance.transact({'to': w3.eth.accounts[0]}).someFunction()

I can access the receipt with w3.eth.getTransactionReceipt(tx_hash), but this does not give me the result. How can I access the result of the call without having to invoke the smart contract twice (as in using both code snippets from above)?

Answer 1

Transactions don't have return values. If the function you're calling returns a value, that's simply ignored. The typical pattern is to log an event from the function and look for that event in the transaction receipt.

Note that if you call a function without sending a transaction (probably what's happening in your first code snippet), then you do get a return value, but the call has no side effects (no state changes or ether transfers).