Smart Contracts using Web3.py

Use the web3.py library for interacting with Ethereum. Its API is derived from the Web3.js JavaScript API and should be familiar to anyone who has used web3.js. The exercise will show how to interact with an already deployed contract on the Ethereum Ropsten Testnet.

Set up environment

Web3.py requires Python 3. It can be installed using pip as follows:

$ pip install web3

Create a new Python file and import the following:

from web3 import Web3, HTTPProvider

We will need HTTPProvider in order to create our connection to the Ropsten Testnet using Infura.io. Now let’s get the necessary Infura.io provider. Go to https://infura.io/ and copy the Ropsten URL.

PROVIDER = "https://rospten.infura.io/<ENDPOINT-ID>"
w3 = Web3(HTTPProvider(PROVIDER))

In order to get a contract instance of an already deployed contract, we will need its address and application binary interface. For this exercise’s purpose, deploy a simple contract storing an array of facts through Remix IDE using MetaMask Ropsten as a provider.

If you do not have ETHt, use the MetaMask faucet: https://faucet.metamask.io/.

Then, copy its address and ABI, and create an instance of the contract. Json library will be needed to the decode the ABI:

CONTRACT_INSTANCE = w3.eth.contract(CONTRACT_ADDRESS, abi=json.loads(ABI))

Writing to the Smart Contract

Now that there is an instance of the contract, create a method for writing facts in the smart contract. It will need the instance, a private key/wallet, the address of the private key/wallet and the fact. The library is not recommended to work with Local Private Keys in Production at the moment, so for the exercise we will enable the unaudited features.

w3.eth.enable_unaudited_features()

Because the contract owner can only add facts to this contract, copy the private key and address. The address will be needed to easily calculate the nonce. We will create a simple transaction, which adds a fact to the contract, sign it with the private key and send it.

Try adding a fact using another private key. RESULT: FAIL

Reading from the Smart Contract

When reading from a Smart Contract, no wallets or private keys are needed. First, create a method which gets a fact by a given index. Then, create a method which gets how many facts are stored in the contract.

$ python3 main.py
Fact 1: The Times 03/Jan/2009 Chancellor on brink of second bailout for banks
Stored facts in the contract: 1

Module

MI4: Module 5: E6