This is the and ardently demanded tutorial about the contract and how to use it with the (UR) to swap or sell tokens. It is also the continuation of the introductory tutorial: . long-awaited Permit2 Uniswap Universal Router How to Buy a Token on the Uniswap Universal Router with Python A Bit of Context After (2015), the standard way to allow a protocol to use your tokens was to it to the token contract and give it an , through a transaction. ERC-20 approve allowance Then, (2020) introduced the concept as a standard to give this allowance using a signed message (thanks to the data signing standard) instead of the approval transaction. This EIP brought some nice like: ERC-2612 permit EIP-712 benefits Simplification: less approvals Gas cost: reduced cost, or even no gas at all Security: approvals include an expiration timestamp and a nonce But because it is built on top of ERC-20, only new (or upgradeable) tokens could enjoy them … Then, (November 2022), which allows signature-based approvals for all tokens, Uniswap released the Permit2 contract along with the Universal Router even if they don’t implement ERC-2612. Tutorial Scope and Target The Permit2 and Universal Router contracts have many features, but we’ll focus only on how to leverage both contracts to swap tokens using signed approvals. More specifically, we’re going to learn how to use the UR function. PERMIT2_PERMIT We finished the introductory tutorial after buying some tokens, so we’ll start from there, and we’ll swap them for some . UNI USDT By the end of this tutorial, you will be able to create a transaction to some tokens from the latest Uniswap router with Python, using . sell Permit2 signature-based approvals Prerequisites This tutorial is intended for developers who are comfortable with Python, the basics of (contracts, ERC20, transactions, gas, etc.), and . Ethereum building transactions with web3.py If not already done, it is also strongly recommended to read the introductory tutorial first ( ) to be familiar with the tools, libraries, and setup we’re going to use here as well. How to Buy a Token on the Uniswap Universal Router with Python Libraries, Tools and Setup In this tutorial, we’re going to use the same libraries and tools as we did in the introductory one, so you can refer to it for installation and setup: Web3.py Uniswap Universal Router Codec Python virtual environment CLI Ganache and npm Node.js You don’t want to learn to swap tokens with actual ones. So you can use either a testnet like or a local fork created with Ganache, which is the solution I’ll use in this tutorial. Sepolia New versions of the Web3.py library and UR SDK have been released in the meantime, so be sure you have the latest ones with the following command: $ pip freeze You should get something like: uniswap-universal-router-decoder==0.9.1
web3==6.11.1 if you have older versions, just update the libraries as usual: $ pip install -U uniswap-universal-router-decoder web3 Launching Ganache We’re going to launch Ganache to fork Ethereum locally at block number 17365005, as we did previously. We’re going to use the same private key and give it 100 ETH (= wei, which is in hexadecimal). 100 * 10**18 0x56bc75e2d63100000 My endpoint address is stored in the environment variable , so the command is: $RPC_ENDPOINT $ ganache --fork.url=$RPC_ENDPOINT --wallet.accounts=0x6c9803151aa5cf420e98dd1afb9db96f0510918cb2249e9858519d47777125c9,0x56bc75e2d63100000 --fork.blockNumber=17365005 Do use the private keys/accounts from these tutorials (or from anywhere) on Mainnet! Everyone could access your fund! NOT Also, if you see some tokens in these accounts on Mainnet (or in any account whose private key is on the web), try to get them: a sweeper bot will steal your gas! NEVER Getting some UNI Tokens We’re going to re-use the script from the previous tutorial and append our new code to have a fully independent script to launch. Make sure the script works and gives you UNI. buy_token.py 368.41333521045374 Swapping tokens And now, the moment you've been waiting for: selling our UNI tokens using the Uniswap Universal Router and the Permit2 contract! Step 1: Approving the Permit2 contract to the UNI one This step is similar to the ERC-20 standard, . It is the step that allows all tokens, even those that don’t implement ERC-2612, to use signature-based approvals. except that you approve the Permit2 contract instead of the router # approve Permit2 to UNI
permit2_address = Web3.to_checksum_address("0x000000000022D473030F116dDEE9F6B43aC78BA3")
permit2_allowance = 2**256 - 1  # max
contract_function = uni_contract.functions.approve(
        permit2_address,
        permit2_allowance
)
trx_params = contract_function.build_transaction(
        {
            "from": account.address,
            "gas": 500_000,
            "maxPriorityFeePerGas": w3.eth.max_priority_fee,
            "maxFeePerGas": 100 * 10**9,
            "type": '0x2',
            "chainId": chain_id,
            "value": 0,
            "nonce": w3.eth.get_transaction_count(account.address),
        }
    )
raw_transaction = w3.eth.account.sign_transaction(trx_params, account.key).rawTransaction
trx_hash = w3.eth.send_raw_transaction(raw_transaction)
print(f"Permit2 UNI approve trx hash: {trx_hash.hex()}") In this tutorial, we give the maximum allowance (2**256 - 1 wei), but depending on your use case, you might want to give less on Mainnet. Checking the allowance we have just approved: print(
        "Permit2 UNI allowance:",
        uni_contract.functions.allowance(account.address, permit2_address).call(),
) You should get: Permit2 UNI allowance: 79228162514264337593543950335 Step 2: Building and Signing the permit message Now, we’re ready to build the swap/sell transaction with a signature-based approval. Let’s start with the permit message. a. Permit2 nonce and allowance For security purposes, the message you need to sign contains a nonce. It is incremented for each permit message you sign. The nonce depends on your account address and on the token and universal router addresses. To know the current Permit2 nonce, allowance, and expiration: permit2_abi = '[{"inputs":[{ ... "type":"function"}]'  # truncated
permit2_contract = w3.eth.contract(address=permit2_address, abi=permit2_abi)
p2_amount, p2_expiration, p2_nonce = permit2_contract.functions.allowance(
        account.address,
        uni_address,
        ur_address
).call()
print(
        "p2_amount, p2_expiration, p2_nonce: ",
        p2_amount,
        p2_expiration,
        p2_nonce,
) For readability sake, I’ve truncated most of the abi, but you can get it from Etherscan, as explained in the previous tutorial. If you have a doubt, you can check the . full tutorial code We haven’t yet given any allowance for Permit2 to the UNI contract, so we expect only zeros: permit2_amount, permit2_expiration, permit2_nonce:  0 0 0 b. Building the signable message The approval message is built as follows: # permit message
allowance_amount = 2**160 - 1  # max/infinite
permit_data, signable_message = codec.create_permit2_signable_message(
        uni_address,
        allowance_amount,
        codec.get_default_expiration(),  # 30 days
        p2_nonce,
        ur_address,
        codec.get_default_deadline(),  # 180 seconds
        chain_id,
    ) Let’s breakdown the command: is the amount you’re happy to approve. Here, we set it to the infinite value: allowance_amount 2**160 - 1 gives the timestamp after which the allowance is not valid anymore. The codec default is at 30 days, but you can set whatever you want in the future: you could set 24h like that: codec.get_default_expiration() codec.get_default_expiration(24 * 3600) : the Permit2 nonce we just got at the previous step. permit2_nonce : the timestamp after which the transaction will not be valid anymore. Default is 180s. codec.get_default_deadline() : 1 for Ethereum, 1337 for Ganache, … chain_id contains the permit message details (basically, the arguments) permit_data codec.create_permit2_signable_message() is the EIP-172 encoded version of the permit message. signable_message c. Signing the message Signing the message is as simple as: signed_message = account.sign_message(signable_message) Step 3: The swap transaction a. Encoding the input data The swap part in the transaction we’re going to send to the Universal Router is very similar to what we learned in the previous tutorial. Now could be a good moment to look into it again and refresh our memory about the method because we’re using it again here: v2_swap_exact_in() # Building the Swap to sell UNI for USDT
usdt_address = Web3.to_checksum_address("0xdac17f958d2ee523a2206206994597c13d831ec7")
usdt_abi = '[{"constant" ... :"Unpause","type":"event"}]'  # truncated
usdt_contract = w3.eth.contract(address=usdt_address, abi=usdt_abi)

amount_in = 100 * 10**18
min_amount_out = 415 * 10**6
path = [uni_address, usdt_address]
encoded_input = (
        codec
        .encode
        .chain()
        .permit2_permit(permit_data, signed_message)
        .v2_swap_exact_in(
                FunctionRecipient.SENDER,
                amount_in,
                min_amount_out,
                path,
                payer_is_sender=True,
        )
        .build(codec.get_default_deadline())
    ) Again, the ABI is truncated: you can get it from Etherscan or from the tutorial full code. Let’s breakdown the command: , , , and have already been explained in the introductory tutorial. codec encode chain() v2_swap_exact_in() build() : Encode the call to the Universal router function , with the previously built arguments: and permit2_permit() PERMIT2_PERMIT permit_data signed_message. : at block , the price of UNI was around $4.19, so we gave a bit of room and asked for a minimum of 415 USDT for our 100 UNI. min_amount_out 17365005 b. Building and sending the transaction And the transaction is built and sent as usual: trx_params = {
        "from": account.address,
        "to": ur_address,
        "gas": 500_000,
        "maxPriorityFeePerGas": w3.eth.max_priority_fee,
        "maxFeePerGas": 100 * 10**9,
        "type": '0x2',
        "chainId": chain_id,
        "value": 0,
        "nonce": w3.eth.get_transaction_count(account.address),
        "data": encoded_input,
}
raw_transaction = w3.eth.account.sign_transaction(trx_params, account.key).rawTransaction
trx_hash = w3.eth.send_raw_transaction(raw_transaction)
print(f"Trx Hash: {trx_hash.hex()}") c. Checking the balances If all goes well, you should have 100 UNI less and a positive USDT balance: # Checking the balances
uni_balance = uni_contract.functions.balanceOf(account.address).call()
print("UNI Balance:", uni_balance / 10**18, "UNI")

usdt_balance = usdt_contract.functions.balanceOf(account.address).call()
print("USDT Balance:", usdt_balance / 10**6, "USDT") UNI Balance: 268.41333521045374 UNI
USDT Balance: 419.503665 USDT d. Checking the new Permit2 allowance # Checking the new Permit2 allowance
p2_amount, p2_expiration, p2_nonce = permit2_contract.functions.allowance(
        account.address,
        uni_address,
        ur_address
).call()
print(
        "p2_amount, p2_expiration, p2_nonce: ",
        p2_amount,
        p2_expiration,
        p2_nonce,
) Now, you should get some values similar to: p2_amount, p2_expiration, p2_nonce:  1461501637330902918203684832716283019655932542975 1700722499 1 As long as and is in the future, you can swap the USDT token; you don’t have to add the method when building your next transaction! p2_amount > 0 p2_expiration permit2_permit(permit_data, signed_message) Conclusion Using the PERMIT2_PERMIT function is the way to authorize the Uniswap Universal Router to swap any ERC-20 tokens, even if they don’t support ERC-2612. The great benefit, compared to the ERC-20 standard, is that you don’t have to approve each and every protocol to the USDT contract if they use the Permit2 contract as well. You just need to include a signed permit message in your transaction. If you made it up to here, congrats !!! You have learnt: How to use the Permit2 contract with the Universal Router to swap a token How to encode and sign a Permit2 permit message In addition, you have: Acquired a better understanding of several important protocols and EIPs Practised what you learnt in the previous tutorial Closing words and disclaimer Now that you have a more advanced knowledge of the UR codec, you can continue to play with the other functions it supports. : I’m the of the Python . This open-source library is a work in progress and does not support all UR functions yet. If you ever experience any issues with it, please open a ticket . And feel free to share what other UR functions you would like to be supported by this SDK. Disclaimer author and maintainer Universal Router SDK here

The code in this story is for educational purposes. The readers are solely responsible for whatever they build with it.

This writer has a vested interest be it monetary, business, or otherwise, with 1 or more of the products or companies mentioned within.

Walkthroughs, tutorials, guides, and tips. This story will teach you how to do something new or how to do something better.

Python: How to Use Permit2 with the Uniswap Universal Router

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