Python web3, how to create route bnb -> intermediary -> token dynamically

Question

Using Python 3.10
web3 V 5.31.1

I currently have functional code in python that can buy/sell tokens via PancakeSwapV2 only if they have liquidity pools BNB/token

def buy_token(tkn_addr: str, amount: float, w3: Web3, pan_contract) -> str:
    # define transaction
    buy_txn = pan_contract.functions.swapExactETHForTokens(
        0,                                                  # slippage (0 == inf)
        [WBNB_ADDR,tkn_addr],                               # trade from BNB --> token
        USER_ADDR,                                          # wallet to send tokens to
        (int(time.time()) + 1000)                           # deadline
    ).buildTransaction({
        'from': USER_ADDR,                                  # wallet to fund trade
        'value': w3.toWei(amount, 'ether'),                 # amount to buy
        'gas': 0,                                           # gas limit
        'gasPrice': w3.toWei(GAS_PRICE,'gwei'),             # gas price
        'nonce': w3.eth.get_transaction_count(USER_ADDR)    # metamask nonce
    })

    # get estimated gas cost of transaction
    try:
        EST_GAS = w3.eth.estimate_gas(buy_txn) + GAS_BUFFER
        buy_txn.update({'gas': EST_GAS})
    # if fails, default to flat rate
    except:
        buy_txn.update({'gas': GAS_LIMIT})

    # sign and send transaction
    signed_txn = w3.eth.account.sign_transaction(buy_txn, private_key=PRIV_KEY)
    txn_hash = w3.toHex(w3.eth.send_raw_transaction(signed_txn.rawTransaction))

    return txn_hash

However, trading sites such as poocoin.app have the functionality to create routes that allow me to buy/sell tokens with non bnb LPs by creating longer route chains, eg bnb -> usdt -> token.

Assuming I already know the intermediary token, this can be accomplished easily by doing

...swapExactTokensForTokens(
    ...
    [WBNB_ADDR, intermediary_addr, tkn_addr]
    ...
)
...

However, given the contract address of the token I am wishing to purchase, how can I query pancakeswap for a valid route from BNB-->...-->token?

I cannot find an example of this functionality in python and am looking for help!

Answer 1

The swap function can take any token as an intermediary. There can be hundreds or thousands of combinations.

Source for _swap function code:

function _swap(uint[] memory amounts, address[] memory path, address _to) internal virtual {
    for (uint i; i < path.length - 1; i++) {
        (address input, address output) = (path[i], path[i + 1]);
        (address token0,) = PancakeLibrary.sortTokens(input, output);
        uint amountOut = amounts[i + 1];
        (uint amount0Out, uint amount1Out) = input == token0 ? (uint(0), amountOut) : (amountOut, uint(0));
        address to = i < path.length - 2 ? PancakeLibrary.pairFor(factory, output, path[i + 2]) : _to;
        IPancakePair(PancakeLibrary.pairFor(factory, input, output)).swap(
            amount0Out, amount1Out, to, new bytes(0)
        );
    }
}

If you are interested in finding out how many swap pairs are on pancake swap, you can query its factory contract's allPairs() method to get the address for LPs.

Currently, there are 1263242 pairs. So you need to call the function 1263242 times.

This is just the hardcore way of making things done. There can be third-party oracles providing this kind of information though.🤔