Python Web3 sign transaction without calling "build_transaction"

Question

I need to sign two different transactions:

token_contract = web3.eth.contract(address=web3.to_checksum_address(TOKEN_ADDRESS), abi=json.loads(erc20_abi))
approve_trx = token_contract.functions.approve(getenv("ROUTER_ADDRESS"), my_out).build_transaction({
    'nonce': web3.eth.get_transaction_count(wallet.address),
    'from': wallet.address,
    'maxFeePerGas': int(trx["maxFeePerGas"], 16) + web3.to_wei(getenv("BRIBE_GWEI"), "gwei") if "maxFeePerGas" in trx else web3.to_wei(getenv("BRIBE_GWEI"), "gwei"),
    'maxPriorityFeePerGas': int(trx["maxPriorityFeePerGas"], 16) + web3.to_wei(getenv("BRIBE_GWEI"), "gwei") if "maxPriorityFeePerGas" in trx else web3.to_wei(getenv("BRIBE_GWEI"), "gwei")
})

and

sell_trx = router_contract.functions.swapExactTokensForETH(my_out, 0, [TOKEN_ADDR, WETH_ADDR], wallet.address, int(time.time()) + 30).build_transaction({
      'nonce': web3.eth.get_transaction_count(wallet.address),
      'from': wallet.address,
      'maxFeePerGas': int(trx["maxFeePerGas"], 16) + web3.to_wei(getenv("BRIBE_GWEI"), "gwei") if "maxFeePerGas" in trx else web3.to_wei(getenv("BRIBE_GWEI"), "gwei"),
      'maxPriorityFeePerGas': int(trx["maxPriorityFeePerGas"], 16) + web3.to_wei(getenv("BRIBE_GWEI"), "gwei") if "maxPriorityFeePerGas" in trx else web3.to_wei(getenv("BRIBE_GWEI"), "gwei")
})

and then I want to sign them with:

signed_approve = web3.eth.account.sign_transaction(approve_trx, getenv("PRIV_KEY"))
signed_sell = web3.eth.account.sign_transaction(sell_trx, getenv("PRIV_KEY"))

However, build_transaction of the sell trx raises the error: execution reverted: TransferHelper: TRANSFER_FROM_FAILED

This is correct since I did not sent the first transaction yet, but is there a way to sing both instead of send the approve and wait for it?

Answer 1

If I understand your issue correctly, signing both the approve and swap transactions simultaneously using the web3 library appears to be impossible. Nevertheless, there are two alternative approaches you can explore:

#1: Smart Contract with Swap Functionality

Create a smart contract featuring a function designed for swapping. This function should inherently include both the approve and swap functionalities within it (ERC821 may help).

#2: Single Approval with a Substantial Token Amount

Basically, you just need to approve only one time with a significantly large token allowance. Once this approval is done, other approvals become unnecessary as the existing allowance is already ample for subsequent swap transactions.