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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)?

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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).

  • Thanks, I'll look at the event documentation of web3.py. Regarding calls: What happens when I call a function where the return value depends on the input? Is the code still executed, but the state changes discarded? – Andreas Unterweger Jan 27 '18 at 18:24
  • Yes, the code is executed locally on the node you're connected to, and then the side effects are discarded. – user19510 Jan 27 '18 at 18:55

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