Which function to use for swapping eth against token on uniswap?

Question

I use the following code to create the swap_transaction:

swap_transaction = router_contract.functions.swapETHForExactTokens(
        0, [weth_address  ,token_address], account_address, int(time.time()) + (10000)

    ).build_transaction(

        {
            'nonce': nonce,
            'gas': 200000,
            'gasPrice': w3.to_wei(gasprice,'gwei'),
            'from': account_address,
            'value': w3.to_wei(ether_amount,'ether')

        }
    )

Here I use the swapETHForExactTokens function but I have also seen the 'swapExactETHForTokensSupportingFeeOnTransferTokens' function. What is the difference between these two and which one should I use? In tutorials and the unofficial uniswap python package the 'swapETHForExactTokens' is used but when I go to etherscan I also see the other function. Thank you

Answer 1

There are some tokens that charge a fee for every transfer. A new function was introduced swapExactETHForTokensSupportingFeeOnTransferTokens because the original function swapExactETHForTokens doesn't know how to deal with those type of tokens.

From the developer documentation

swapExactTokensForTokensSupportingFeeOnTransferTokens: Identical to swapExactTokensForTokens, but succeeds for tokens that take a fee on transfer.