Programming an Ethereum Smart Contract with Vyper

and deploying it with MetaMask and MyEtherWallet (MEW)


Introduction

Ethereum has a vast amount of developer resources available, such that for a beginner its hard to know where to start.

If you go with the recommended approach you'd probably choose Solidity along with either JavaScript or golang. JavaScript and golang are traditionally the most well supported languages for the developer tooling around Ethereum. The original Ethereum node software, geth, is written in golang. There's a version of the Solidity smart contract compiler, solcjs, written in JavaScript, and available as a Node.js NPM package.

The other Smart Contract language that is supported by the Ethereum Foundation is Vyper. Vyper is a python-like language that was designed for security, simplicity and testability. In my view this makes Vyper ideal for a beginner. Vyper foregoes some of the power of Solidity (for example, class inheritance and function modifiers) in order to uphold the Vyper principles of being simple and secure.

In this article I'll be stepping you through creating a smart contract with the Vyper programming language, deploying it to the Ethereum test network, and then interacting with the contract - calling its two externally accessible functions. The contract deployment and contract interaction are achieved using two in-browser tools - MyEtherWallet (MEW) and MetaMask.


Background reading

Before you start I would highly recommend this article to get up to speed with the Ethereum ecosystem as a whole.
https://medium.com/@mattcondon/getting-up-to-speed-on-ethereum-63ed28821bbe
Its from 2017 but the content is still valid today.

Along with the Ethereum whitepaper, which is a must-read if you're a developer:
https://github.com/ethereum/wiki/wiki/White-Paper


Installing the Vyper compiler

There's a number of options available to you as documented here:
https://vyper.readthedocs.io/en/latest/installing-vyper.html

One of the simplest options is to install it using python's package manager (pip). I'll start by ensuring you have the python development environment setup correctly.


Installing a recent version of Python and Pip with Pyenv

If you don't have already have a recent version of Python installed, or don't even know, I would highly recommend that you start by installing pyenv. Pyenv is a tool for python version management.

I followed the Pyenv installation instructions for a MacOS installation via homebrew, but there are instructions for other OSes given.

After installing pyenv, you can install the latest version of python with

% pyenv install 3.8.0

You will now have an up to date version of both python and pip available from your terminal command prompt.

Check what version of python is installed:
% python -V

Output:
Python 3.8.0

Check what version of pip is installed:
% pip -V

Output:
pip 19.2.3 from /Users/hg/.pyenv/versions/3.8.0/lib/python3.8/site-packages/pip (python 3.8)


Installing the Vyper compiler

Once you have python and pip installed aready, you can install the Vyper compiler with:

% pip install vyper

Check what version of Vyper is installed:

% vyper --version

Output:
0.1.0b16+commit.5e4a94a


Writing a smart contract in Vyper

For the first contract, we're going to keep things very simple and go with a public storage contract.
All the contract does is stores a single number in its storage area, and implements two public accessor functions - a getter and a setter.

Use a text editor and open a new file named storage.vy

Note that vy is the file extension for Vyper source code.

Here's the Vyper code:

stored_data: uint256

@public
def set(new_value : uint256):
    self.stored_data = new_value

@public
@constant
def get() -> uint256:
    return self.stored_data

Line 1 declares a variable named 'stored_data' of type 'uint256'. This is the only thing that the contract will store on the blockchain.

Line 3 (def set...) declares the public set function, to update the 'stored_data' variable.

Line 9 (def get...) declares the public get function, to get the 'stored_data' variable.

Its important to note that these two functions get and set are marked public and so can be 'called' by anyone - by using Ethereum client software like MyEtherWallet, or programmatically via the web3 or by other library.

Calling a public function on a smart contract actually consists of sending a transaction to the deployed contract's address - but this is an implementation detail hidden that is hidden by client software or libraries.

Further reading about the structure of contracts in Vyper:
https://vyper.readthedocs.io/en/latest/structure-of-a-contract.html


Compiling the Vyper smart contract

To compile the contract into EVM bytecode:

% vyper -f bytecode storage.vy > storage.bin

To generate the ABI for the contract:

% vyper -f abi storage.vy > storage.abi

The ABI is a JSON format interface specification file, and it is needed to be able to interact with the smart contract (i.e. call its functions).


Deploy the smart contract to the Rinkeby test network

Install MetaMask

If you haven't already, now is a good time to install the MetaMask plugin/extension into your browser - https://metamask.io/
Setup MetaMask with a seed phrase and it will create an in-browser Ethereum wallet that you can use for real transactions, or in this case, test transactions.

When MetaMask is setup, at the top right you should see the network dropdown.

Change this to 'Rinkeby Test Network'. Press the DEPOSIT button, then press the GET ETHER button under the TEST FAUCET option.

After opening https://www.rinkeby.io/#stats use the 'Crypto Faucet' button on the left and follow the instructions. The minimum option (3 ETH) will be more than enough for test purposes and to deploy this contract.

The second thing you'll need to do is to setup MyEtherWallet (MEW). This will let you deploy and interact with your smart contract.


Setup MyEtherWallet (MEW)

Go to https://www.myetherwallet.com and setup the seed phrase.

Now you'll need to give MyEtherWallet (MEW) access to your MetaMask wallet, which contains the 3 ETH for testing.

Go to https://www.myetherwallet.com/access-my-wallet and press the browser extension option.

It should come up with a 'Access via MetaMask' window, and you'll need to grant it permission. Press the 'Access My Wallet' button.


Deploy the smart contract

Inside MyEtherWallet (MEW), press the left hand side option 'Contract' then 'Deploy Contract'.

From the terminal, issue this command:

% cat storage.bin

Output:
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

Using the mouse, copy the contents of the file from the terminal prompt.

Inside MEW, paste the copied text contents into the bytecode field.

On the terminal again, issue this command:

% cat storage.abi

Output:
[{"name": "set", "outputs": [], "inputs": [{"type": "uint256", "name": "new_value"}], "constant": false, "payable": false, "type": "function", "gas": 35315}, {"name": "get", "outputs": [{"type": "uint256", "name": "out"}], "inputs": [], "constant": true, "payable": false, "type": "function", "gas": 1181}]

Using the mouse, copy the contents of the file from the terminal prompt.

Inside MEW, paste the copied text contents into the ABI/JSON field.

Enter a name for your contract, e.g. 'storage1'.

Press the 'Sign Transaction' button.

MetaMask will open up a window where you'll be able to confirm the deployment of your contract.

You'll see the gas fee and total.
You can press on the data tab and you'll see the bytecode that you pasted earlier.

Press the Confirm button to proceed with the deployment of your contract.

You may need to wait several seconds for the transaction to be confirmed at this point.
MetaMask should give you a successful deployment popup and link you to the transaction on etherscan.io.
E.g.
https://rinkeby.etherscan.io/tx/0x36663b338ab0eaa7d7cdd91aa5abacdc273757ff56b81221d76a2ff0aedc9860

Here you can see the address of your newly deployed contract. In my case it is here:
https://rinkeby.etherscan.io/address/0x7baad2f634d6bde84916e7d6db40ca2e502eaff6


Side note on contract addresses

Contract addresses are calculated deterministically, based on the account that created it. They are based on the nonce value of your account. So it is possible to know in advance what the contract address will be, if necessary.

Details here: https://ethereum.stackexchange.com/questions/760/how-is-the-address-of-an-ethereum-contract-computed


Interacting with the smart contract

Now that your contract is deployed, its time to interact with it - call its get and set functions.

The get function can be called 'for free' - it doesn't cost any gas, because its just returning the current state of the blockchain.
Calling the 'set' fuction however must be paid for in gas, because by calling it you are actually changing the current state of the blockchain.

Go back to MEW and navigate to the left hand side 'Contract' option then 'Interact with Contract'.

Paste your newly created contract address into the 'Contract Address' field.

In the ABI/JSON field, past your ABI file contents.

Press the Continue button.

There's a dropdown which shows 'Select an item'. This lists all of the public functions on the smart contract. There's only two available - get and set.

Choose get first, it will execute immediately and show the result being 0. 0 is the default value for uninitialised storage in Ethereum.

Now choose 'set' from the dropdown.

Enter the required value into the new_value field, e.g. 88.

Leave the Value in ETH as 0.

Press the 'Write' button.

MetaMask will open up a window where you'll be able to confirm your contract interaction.
What you're actually doing here is sending a transaction to the contract with a value in the data field - the data specifies to the EVM which function is being called (set) and the value of the parameter to it (88).

Press the confirm button.

You may need to wait several seconds for the transaction to be confirmed.

MetaMask should give you a confirmed transaction popup and link you to the transaction on etherscan.io.
E.g.
https://rinkeby.etherscan.io/tx/0x42af3501fff6ddd67a97cc52c94565f149f4d7f985d223523cebae2efc693bbb

Press the "Press to see more" link at the bottom of this page on etherscan.io to see the detail of the function call you made.

Go back to MEW and this time select the 'get' from the dropdown.

Confirm that the value you set is now reflected by the get.

You have now successfully deployed an Ethereum smart contract written in Vyper, and interacted with it using MetaMask and MyEtherWallet (MEW).

Congratulations!


Final notes - can you modify a smart contract?

Note that once a smart contract is deployed, you can't modify it; it is immutable and exists on the Ethereum blockchain forever.

If you need to modify or update the contract, you'll need to deploy the updated code to a new contract address.

There are various techniques for handling contract "upgrades" including migrating the existing storage. There's a good article here on this topic: https://mixbytes.io/blog/storage-upgradable-ethereum-smart-contracts

Note also that a contract can destroy itself (along with its storage), via the selfdestruct() function, as documented here: https://vyper.readthedocs.io/en/v0.1.0-beta.15/built-in-functions.html#selfdestruct

Once destroyed, the contract and its storage are removed from that block onwards. After destruction, it is technically still possible to see the historical contract bytecode and storage, which exists on earlier blocks of the Ethereum blockchain.


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