How to estimate gas without balance?

Question

So, I'm making a private network and deploying some contracts. The contract has a payable method which means I have to put value when calling the method. I want to get the estimated cost to call the method using this function in web3.py

contract = w3.eth.contract(address=address, abi=abi)
transaction = contract.functions.someMethod().buildTransaction({
    'from': someUserAddress,
    'value': someValue
})
print(w3.eth.estimate_gas(transaction))

This works fine for user that has balance in their wallet. But, for user that has 0 balance in their wallet, this method is not working. This error must be show up

ValueError: {'code': -32000, 'message': 'err: insufficient funds for gas * price + value: address <someUserAddress> have 0 want <someValue> (supplied gas xxxxxxx)'}

I need to estimate gas first so the user can know how much they must top up the balance to call the method and not blindly to up the balance. Is there any way to do this?

Answer 1

I guess you are bootstrapping private Ethereum network.

First of all, when you have payable function, you are not forced to send value into it, but you are able to.

Secondly, you have to pay gas for your transaction and that is probably your issue. If your user has 0 balance, he/she is not able to send transaction even when transferred value is 0.

You can find more how to estimate gas usage in web3.py here: https://web3py.readthedocs.io/en/stable/contracts.html?highlight=gas%20estimate#web3.contract.ContractFunction.estimateGas