I would like to call a smart contract functions using Web3 Python using the Infura mainnet node instead of a local node. I understand that I need to import the private key and use it to sign the transaction but I cannot see any examples for doing this with calling smart contracts.

The code I have is as follows but it fails as there is no wallet attached:

from web3 import Web3
import json
from web3.providers.rpc import HTTPProvider
contractAddress = '0xabcd'
web3 = Web3(HTTPProvider('https://mainnet.infura.io'))
with open('abcd.abi', 'r') as abi_definition:
    abi = json.load(abi_definition)
print (web3.eth.blockNumber)

fContract = web3.eth.contract(abi,contractAddress)

print ("Test",fContract.transact().test("0xabcd",100000))


ValueError: {u'message': u'gas required exceeds allowance or always failing transaction', u'code': -32000}
  • 2
    If you are getting familiar with Blockchain you should use a testnet such as Ropsten or Rinkeby. Using mainnet (mainnet.infura.io) will cost you Ether if you have a wallet (such as MetaMask) setup in your browser or fail as you do not have ether to pay for gas.
    – Chim
    Commented Jan 15, 2018 at 11:56
  • If you are happy to read a JavaScript code and understand the steps involve with signing a transaction, you could refer npmjs.com/package/web3js-raw & github.com/fidenz-chim/fundsplitter_web3jsraw.git
    – Chim
    Commented Jan 15, 2018 at 11:59

1 Answer 1


Check out the example of how to invoke a contract using a local key in the docs.

Docs for some of the key methods are here:

Roughly, you're going to want to:

txn = fContract.functions.test("0xabcd", 100000).build_transaction()
signed = web3.eth.account.sign_transaction(txn, privateKey)
txn_hash = web3.eth.send_raw_transaction(signed.rawTransaction)
  • 1
    Hi carver, thank you. Also you missed private key: signed = web3.eth.account.signTransaction(txn, privateKey)
    – Russo
    Commented May 21, 2018 at 5:20
  • Hi carver, would you explain a bit in detail about how to use these functions? I tried them but they are not working. Would you have a look here: ethereum.stackexchange.com/questions/49038/… ? Thank you
    – Russo
    Commented May 21, 2018 at 11:03

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