Deploy and Interact with a Smart Contract using Web3.py

Web3.py is a Python library that allows developers to interact with Ethereum-based blockchains with Python. Rootstock has an Ethereum-like API available that is fully compatible with Ethereum-style JSON-RPC invocations. Therefore, developers can leverage this compatibility and use the Web3.py library to interact with Rootstock similar to how developers interact with smart contracts on Ethereum.

In this guide, you'll learn how to use the Web3.py library to deploy and interact with smart contracts on Rootstock.

Interact with Rootstock using Rust

See tutorial on how to interact with Rootstock using Rust

Prerequisites

• A testnet account with tRBTC funds.
  • Get tRBTC.
• An API KEY from the Rootstock RPC Service.
• Set up the project
• A Solidity Compiler installed -> see solidity compiler installation instructions

Set up the project and install dependencies:

# create a directory for the project
mkdir web3-python-guide && cd web3-python-guide

# install python 3.10
brew install python@3.10

# set up the development virtual environment
python3.10 -m venv env
source env/bin/activate

# install dependencies
pip install Web3 py-solc-x

Solidity compiler installation instructions for MacOs:

brew install solc-select
solc-select use 0.8.19 --always-install

solc --version
# Version: 0.8.19+commit.7dd6d404.Darwin.appleclang

Set Up Secrets for the Project

We will be using sensitive data that doesn’t have to be stored in the code, and instead we will store them in a .env file.

For that, first lets install the package to read data from the .env file:

pip install python-dotenv

Then, we will create a .env file and add the secrets:

touch .env

add the following variables to the file:

Replace YOUR_APIKEY with the API key from your dashboard.

# get this YOUR_APIKEY from the Rootstock RPC Service.
RPC_PROVIDER_APIKEY = '{YOUR_APIKEY}'

# this is the private key of the account from which you will deploy the contract
ACCOUNT_PRIVATE_KEY = '{YOUR_PRIVATE_KEY}'

Deploy a smart contract

Write the smart contract

The contract to be compiled and deployed in the next section is a simple contract that stores a message, and will allow for setting different messages by sending a transaction.

You can get started by creating a file for the contract:

touch Greeter.sol

Next, add the Solidity code to the file:

// SPDX-License-Identifier: MIT

pragma solidity >0.5.0;

contract Greeter {
    string public greeting;

    constructor() public {
        greeting = 'Hello';
    }

    function setGreeting(string memory _greeting) public {
        greeting = _greeting;
    }

    function greet() view public returns (string memory) {
        return greeting;
    }
}

The constructor function, which runs when the contract is deployed, sets the initial value of the string variable stored on-chain to “Hello”. The setGreeting function adds the _greeting provided to the greeting, but a transaction needs to be sent, which modifies the stored data. Lastly, the greet function retrieves the stored value.

Compile the smart contract

We will create a script that uses the Solidity compiler to output the bytecode and interface (ABI) for the Greeter.sol contract. To get started, we will create a compile.py file by running:

touch compile.py

Next, we will create the script for this file and complete the following steps: Import the solcx package, which will compile the source code Compile the Greeter.sol contract using the solcx.compile_files function Export the contract's ABI and bytecode

Code and paste the code below into compile.py;

import solcx
solcx.install_solc('0.8.19')

# Compile contract
temp_file = solcx.compile_files(
    'Greeter.sol',
    output_values=['abi', 'bin'],
    solc_version='0.8.19'
)

# Export contract data
abi = temp_file['Greeter.sol:Greeter']['abi']
bytecode = temp_file['Greeter.sol:Greeter']['bin']

You can now run the script to compile the contract:

python compile.py

Deploy the smart contract

With the script for compiling the Greeter.sol contract in place, you can then use the results to send a signed transaction that deploys it. To do so, you can create a file for the deployment script called deploy.py:

touch deploy.py

Next, you will create the script for this file and complete the following steps:

1. Add imports, including Web3.py and the ABI and bytecode of the Greeter.sol contract
2. Set up the Web3 provider

In order to set up the Web3 Provider, we have to read the environment variables that we previously added to the .env file.

# Add the Web3 Provider
RPC_PROVIDER_APIKEY = os.getenv('RPC_PROVIDER_APIKEY')
RPC_PROVIDER_URL = 'https://rpc.testnet.rootstock.io/' + RPC_PROVIDER_APIKEY
web3 = Web3(Web3.HTTPProvider(RPC_PROVIDER_URL))

3. Define the account_from. The private key is required to sign the transaction. Note: This is for example purposes only. Never store your private keys in your code

# Set the default account
PRIVATE_KEY = os.getenv('ACCOUNT_PRIVATE_KEY')
account_from = {
    'private_key': PRIVATE_KEY,
    'address': web3.eth.account.from_key(PRIVATE_KEY).address
}

4. Create a contract instance using the web3.eth.contract function and passing in the ABI and bytecode of the contract
5. Set the gas price strategy using the web3.eth.set_gas_price_strategy function, which will allow us to fetch the gasPrice from the RPC Provider. This is important because otherwise the Web3 library will attempt to use eth_maxPriorityFeePerGas and eth_feeHistory RPC methods, which are only supported by post-London Ethereum nodes.
6. Build a constructor transaction using the contract instance. You will then use the build_transaction function to pass in the transaction information including the from address and the nonce for the sender. To get the nonce you can use the web3.eth.get_transaction_count function
7. Sign the transaction using the web3.eth.account.sign_transaction function and pass in the constructor transaction and the private_key of the sender
8. Using the signed transaction, you can then send it using the web3.eth.send_raw_transaction function and wait for the transaction receipt by using the web3.eth.wait_for_transaction_receipt function

Code and paste the code below into deploy.py;

from compile import abi, bytecode
from web3 import Web3
from web3.gas_strategies.rpc import rpc_gas_price_strategy
from dotenv import load_dotenv
import os

load_dotenv()

# Add the Web3 Provider
RPC_PROVIDER_APIKEY = os.getenv('RPC_PROVIDER_APIKEY')
RPC_PROVIDER_URL = 'https://rpc.testnet.rootstock.io/' + RPC_PROVIDER_APIKEY
web3 = Web3(Web3.HTTPProvider(RPC_PROVIDER_URL))


# Set the default account
PRIVATE_KEY = os.getenv('ACCOUNT_PRIVATE_KEY')
account_from = {
    'private_key': PRIVATE_KEY,
    'address': web3.eth.account.from_key(PRIVATE_KEY).address
}

print("Attempting to deploy from account: ", account_from['address'])

# Create contract instance
Greeter = web3.eth.contract(abi=abi, bytecode=bytecode)

# Set the gas price strategy
web3.eth.set_gas_price_strategy(rpc_gas_price_strategy)

# Build the transaction
construct_txn = Greeter.constructor().build_transaction({
    'from': account_from['address'],
    'nonce': web3.eth.get_transaction_count(account_from['address']),
    'gasPrice': web3.eth.generate_gas_price()
})

# Sign the transaction that deploys the contract
signed_txn = web3.eth.account.sign_transaction(construct_txn, account_from['private_key'])

# Send the transaction that deploys the contract
txn_hash = web3.eth.send_raw_transaction(signed_txn.rawTransaction)

# Wait for the transaction to be mined, and get the transaction receipt
txn_receipt = web3.eth.wait_for_transaction_receipt(txn_hash)
print(f"Transaction successful with hash: { txn_receipt.transactionHash.hex() }")
print(f"Contract deployed at address: { txn_receipt.contractAddress }")

Now you can run the script and get the result.

python deploy.py

>> Attempting to deploy from account:  0x3b32a6463Bd0837fBF428bbC2A4c8B4c022e5077
>> Transaction successful with hash: 0x98a256c106bdb65e4de6a267e94000acdfe0d6f23c3dc1444f14dccf00713a69
>> Contract deployed at address: 0xba39f329255d55a0276c695111b2edc9250C2341

Note: Save the contract address, as we will use it later in the guide.

Interact with a smart contract

Read Contract Data (Call Methods)

Call methods are the type of interaction that don't modify the contract's storage (change variables), meaning no transaction needs to be sent. They simply read various storage variables of the deployed contract.

To get started, you can create a file and name it getMessage.py:

touch getMessage.py

Then you can take the following steps to create the script:

1. Add imports, including Web3.py and the ABI of the Greeter.sol contract
2. Set up the Web3 provider and replace YOUR_APIKEY
3. De fine the contract_address of the deployed contract
4. Create a contract instance using the web3.eth.contract function and passing in the ABI and address of the deployed contract
5. Using the contract instance, you can then call the greet function

Code and paste the code below into getMessage.py;

from compile import abi
from web3 import Web3
from dotenv import load_dotenv
import os

load_dotenv()

# Add the Web3 Provider
RPC_PROVIDER_APIKEY = os.getenv('RPC_PROVIDER_APIKEY')
RPC_PROVIDER_URL = 'https://rpc.testnet.rootstock.io/' + RPC_PROVIDER_APIKEY
web3 = Web3(Web3.HTTPProvider(RPC_PROVIDER_URL))


# Create address variable (use the address of the contract you just deployed)
contract_address = '0xba39f329255d55a0276c695111b2edc9250C2341'

print(f"Making a call to contract at address: { contract_address }")

# Create contract instance
Greeter = web3.eth.contract(address=contract_address, abi=abi)

# Call the contract
call_result = Greeter.functions.greet().call()
print(f"Contract returned: { call_result }")

If successful, the response will be displayed in the terminal:

python getMessage.py

>> Making a call to contract at address: 0xba39f329255d55a0276c695111b2edc9250C2341
>> Contract returned: Hello

Write data to the contract (Write Methods)

Write methods are the type of interaction that modify the contract's storage (change variables), meaning a transaction needs to be signed and sent. In this section, you'll create the script to change the text stored in the Greeter contract.

To get started, you can create a file for the script and name it setMessage.py:

touch setMessage.py

Open the setMessage.py file and take the following steps to create the script:

1. Add imports, including Web3.py and the ABI of the Incrementer.sol contract
2. Set up the Web3 provider
3. Define the account_from variable, including the private_key, and the contract_address of the deployed contract. The private key is required to sign the transaction. Note: This is for example purposes only. Never store your private keys in your code
4. Create a contract instance using the web3.eth.contract function and passing in the ABI and address of the deployed contract
5. Set the gas price strategy using the web3.eth.set_gas_price_strategy function, which will allow us to fetch the gasPrice from the RPC Provider. This is important because otherwise the Web3 library will attempt to use eth_maxPriorityFeePerGas and eth_feeHistory RPC methods, which are only supported by post-London Ethereum nodes.
6. Build the setGreeting transaction using the contract instance and passing in the new message. You'll then use the build_transaction function to pass in the transaction information including the from address and the nonce for the sender. To get the nonce you can use the web3.eth.get_transaction_count function
7. Sign the transaction using the web3.eth.account.sign_transaction function and pass in the setGreeting transaction and the private_key of the sender
8. Using the signed transaction, you can then send it using the web3.eth.send_raw_transaction function and wait for the transaction receipt by using the web3.eth.wait_for_transaction_receipt function

Code and paste the code below into setMessage.py;

from compile import abi
from web3 import Web3
from web3.gas_strategies.rpc import rpc_gas_price_strategy
from dotenv import load_dotenv
import os

load_dotenv()

# Add the Web3 Provider
RPC_PROVIDER_APIKEY = os.getenv('RPC_PROVIDER_APIKEY')
RPC_PROVIDER_URL = 'https://rpc.testnet.rootstock.io/' + RPC_PROVIDER_APIKEY
web3 = Web3(Web3.HTTPProvider(RPC_PROVIDER_URL))


# Set the default account
PRIVATE_KEY = os.getenv('ACCOUNT_PRIVATE_KEY')
account_from = {
    'private_key': PRIVATE_KEY,
    'address': web3.eth.account.from_key(PRIVATE_KEY).address
}

# Create address variable
contract_address = '0xba39f329255d55a0276c695111b2edc9250C2341'

# Create contract instance
Greeter = web3.eth.contract(address=contract_address, abi=abi)

# Set the gas price strategy
web3.eth.set_gas_price_strategy(rpc_gas_price_strategy)

# Build the transaction
txn = Greeter.functions.setGreeting('Hello, World!').build_transaction({
    'from': account_from['address'],
    'nonce': web3.eth.get_transaction_count(account_from['address']),
    'gasPrice': web3.eth.generate_gas_price()
})

# Sign the transaction
signed_txn = web3.eth.account.sign_transaction(txn, account_from['private_key'])

# Send the transaction
txn_hash = web3.eth.send_raw_transaction(signed_txn.rawTransaction)
txn_receipt = web3.eth.wait_for_transaction_receipt(txn_hash)

print(f"Transaction successful with hash: { txn_receipt.transactionHash.hex() }")

If successful, the transaction hash will be displayed in the terminal.

python setMessage.py

>> Transaction successful with hash: 0x95ba4e13269aba8e51c3037270c0ee90f4872c36e076fc94e51226c1597f6d86

You can now run the getMessage.py script to get the new value stored at the contract.

python getMessage.py

>> Making a call to contract at address: 0xba39f329255d55a0276c695111b2edc9250C2341
>> Contract returned: Hello, World!

Sending transactions

Here you will understand how to check the balance of an account, and how to send tRBTC from one account to another.

Check the balance of an account

Here you will create a script that checks the balance of an account. First, start by creating a file for the script.

touch balances.py

Next, you will create the script for this file and complete the following steps:

1. Set up the Web3 provider
2. Define the address_from and address_to variables
3. Get the balance for the accounts using the web3.eth.get_balance function and format the 3. results using the web3.from_wei

Code and paste the code below into balances.py;

from web3 import Web3
from dotenv import load_dotenv
import os

load_dotenv()


# Add the Web3 Provider
RPC_PROVIDER_APIKEY = os.getenv('RPC_PROVIDER_APIKEY')
RPC_PROVIDER_URL = 'https://rpc.testnet.rootstock.io/' + RPC_PROVIDER_APIKEY
web3 = Web3(Web3.HTTPProvider(RPC_PROVIDER_URL))


# Create address variables
address_from = '0x3b32a6463Bd0837fBF428bbC2A4c8B4c022e5077'
address_to = '0xcff73226883c1cE8b3bcCc28E45c3c92C843485c'

# Get the balance of the sender
balance_from = web3.from_wei(web3.eth.get_balance(address_from), 'ether')
print(f"Balance of sender address {address_from}: { balance_from } TRBTC")

# Get the balance of the receiver
balance_to = web3.from_wei(web3.eth.get_balance(address_to), 'ether')
print(f"Balance of receiver address {address_to}: { balance_to } TRBTC")

Run the script:

python balances.py

# >> Balance of sender address 0x3b32a6463Bd0837fBF428bbC2A4c8B4c022e5077: 0.192538506119378425 TRBTC
# >> Balance of receiver address 0xcff73226883c1cE8b3bcCc28E45c3c92C843485c: 0.407838671951567233 TRBTC

Send TRBTC

Here you will create a script to send tRBTC from one account to another. First, start by creating a file for the script.

touch transaction.py

Next, you will create the script for this file and complete the following steps:

1. Add imports, including Web3.py and the rpc_gas_price_strategy, which will be used in the following steps to get the gas price used for the transaction
2. Set up the Web3 provider
3. Define the account_from, including the private_key, and the address_to variables. The private key is required to sign the transaction. Note: This is for example purposes only. Never store your private keys in your code
4. Use the Web3.py Gas Price API to set a gas price strategy. For this example, you'll use the imported rpc_gas_price_strategy. This is important because otherwise the Web3 library will attempt to use eth_maxPriorityFeePerGas and eth_feeHistory RPC methods, which are only supported by post-London Ethereum nodes.
5. Create and sign the transaction using the web3.eth.account.sign_transaction function. Pass in the nonce, gas, gasPrice, to, and value for the transaction along with the sender's private_key. To get the nonce you can use the web3.eth.get_transaction_count function and pass in the sender's address. To predetermine the gasPrice you'll use the web3.eth.generate_gas_price function. For the value, you can format the amount to send from an easily readable format to Wei using the web3.to_wei function
6. Using the signed transaction, you can then send it using the web3.eth.send_raw_transaction function and wait for the transaction receipt by using the web3.eth.wait_for_transaction_receipt function

Code and paste the code below into transaction.py;

from web3 import Web3
from web3.gas_strategies.rpc import rpc_gas_price_strategy
from dotenv import load_dotenv
import os

load_dotenv()

# Add the Web3 Provider
RPC_PROVIDER_APIKEY = os.getenv('RPC_PROVIDER_APIKEY')
RPC_PROVIDER_URL = 'https://rpc.testnet.rootstock.io/' + RPC_PROVIDER_APIKEY
web3 = Web3(Web3.HTTPProvider(RPC_PROVIDER_URL))



# Set the default account
PRIVATE_KEY = os.getenv('ACCOUNT_PRIVATE_KEY')
account_from = {
    'private_key': PRIVATE_KEY,
    'address': web3.eth.account.from_key(PRIVATE_KEY).address
}
address_to = '0xcff73226883c1cE8b3bcCc28E45c3c92C843485c'

print(f"Attempting to send transaction from { account_from['address'] } to { address_to }")

# Set the gas price strategy
web3.eth.set_gas_price_strategy(rpc_gas_price_strategy)

# Build the transaction
txn = {
    'to': address_to,
    'value': web3.to_wei(0.0001, 'ether'),
    'gas': 21000,
    'gasPrice': web3.eth.generate_gas_price(),
    'nonce': web3.eth.get_transaction_count(account_from['address'])
}

# Sign the transaction
signed_txn = web3.eth.account.sign_transaction(txn, account_from['private_key'])

# Send the transaction
txn_hash = web3.eth.send_raw_transaction(signed_txn.rawTransaction)

# Wait for the transaction to be mined, and get the transaction receipt
txn_receipt = web3.eth.wait_for_transaction_receipt(txn_hash)
print(f"Transaction successful with hash: { txn_receipt.transactionHash.hex() }")

Run the script:

python transaction.py

Attempting to send transaction from 0x112621448Eb148173d5b00edB14B1f576c58cCEE to 0xcff73226883c1cE8b3bcCc28E45c3c92C843485c
Transaction successful with hash: 0x79ab8be672b0218d31f81876c34321ee7b08e6a4ec8bfff5249f70c443cbce00

Summary

In this guide, we learnt how to use the Web3.py library to deploy, interact with a smart contract and send transactions on Rootstock.

Troubleshooting

• When deploying a smart contract, or when trying to interact with it, you may receive the “method not found” message:

    web3.exceptions.MethodUnavailable: {'code': -32601, 'message': 'The method eth_sendTransaction does not exist/is not available. See available methods at https://dev.rootstock.io/developers/rpc-api/methods'}

• Note: The cause of the error on the deployment is that the Web3.py module is set to use the private keys of the RPC provider (Hosted Keys), which is a legacy way to use accounts, and is not supported by modern RPC providers, as they do not store private keys.
• Methods like web3.eth.send_transaction do not work with RPC providers, because they rely on a node state and all modern nodes are stateless, which underneath make JSON-RPC calls to methods like eth_accounts and eth_sendTransaction. You must always use local private keys when working with nodes hosted by someone else.
• If unfamiliar, note that you can export your private keys from Metamask and other wallets. Remember to never share your private keys, and do not put it on your code or repository.
• In order to successfully deploy the contract, the developer needs to set up Web3.py to use his Local Private Keys, and to build and pre-sign the transaction before sending it, so the module uses eth_sendRawTransaction instead.
• To allow Web3.py to use the local keys, we have to use the Signing middleware to add the Private Key to the signing keychain.

    import os
    from eth_account import Account
    from eth_account.signers.local import LocalAccount
    from web3 import Web3, EthereumTesterProvider
    from web3.middleware import construct_sign_and_send_raw_middleware

    w3 = Web3(EthereumTesterProvider())

    private_key = os.environ.get("PRIVATE_KEY")
    assert private_key is not None, "You must set PRIVATE_KEY environment variable"
    assert private_key.startswith("0x"), "Private key must start with 0x hex prefix"

    account: LocalAccount = Account.from_key(private_key)
    w3.middleware_onion.add(construct_sign_and_send_raw_middleware(account))

    print(f"Your hot wallet address is {account.address}")

• Now you can use web3.eth.send_transaction(), Contract.functions.xxx.transact() functions with your local private key through middleware and you no longer get the error "ValueError: The method eth_sendTransaction does not exist/is not available.

• Web3.js will try to use these methods because the Ethereum London fork introduced maxFeePerGas and maxPriorityFeePerGas transaction parameters that can be used instead of gasPrice, which Rootstock uses. For that reason, we have to define Web3’s behavior for populating the gas price. This is done using a “Gas Price Strategy” - a method which takes the Web3 object and a transaction dictionary and returns a gas price (denominated in wei).
• A gas price strategy is implemented as a python method with the following signature, and by setting the gas price strategy by calling set_gas_price_strategy().
  • Setting a specific gas price:

  from web3 import Web3, HTTPProvider

  # specify Gas Price in wei
  GAS_PRICE = 60000000

  def gas_price_strategy(web3, transaction_params=None):
      return GAS_PRICE

  # set the gas price strategy
  w3.eth.set_gas_price_strategy(gas_price_strategy)

• Using eth_gasPrice method:
• Makes a call to the JSON-RPC eth_gasPrice method which returns the gas price configured by the connected Ethereum node.

    from web3.gas_strategies.rpc import rpc_gas_price_strategy
    from web3 import Web3, HTTPProvider

    RPC_PROVIDER = 'https://rpc.testnet.rootstock.io/{API_KEY}'

    w3 = Web3(HTTPProvider(RPC_PROVIDER))
    w3.eth.set_gas_price_strategy(rpc_gas_price_strategy)

    gasPrice = w3.eth.generate_gas_price()

    print('gasPrice: ', gasPrice)

Resources

• Web3.py: Gas Price Strategy
• Infura: eth_accounts
• Infura: eth_sendTransaction
• Web3.py: Working with Local Private Keys
• Web3.py: Contract Deployment Example
• Web3.py: Sign a Contract Transaction
• Web3.py: Setting up an RPC Provider