Develop an Ethereum smart contract with Flask and Vyper

Posted on Sun 27 June 2021 in ETH

This is an updated version of Neha Ghogales blog post How to develop Ethereum contract using Python Flask?


  • Python 3.9
  • Web3 (version 5.x)
  • Node (version 10) - Used for Ganache
  • Ganache (as personal Etherum blockchain for testing)


Create a Python virtual environment and install dependencies:

$ python -m venv smart-contract
$ cd smart-contract
$ bin/pip install web3 flask flask-restful marshmallow vyper

Install Ganache Etherum development environment:

$ npm install -g ganache-cli


Instead of Solidity I'm using Vyper for the smart contract here. We create a file called users.vy:

# @version ^0.2.0

struct User_obj:
    name: String[50]
    gender: int128

MALE: constant(int128) = 0
FEMALE: constant(int128) = 1
OTHER: constant(int128) = 2

user: User_obj

def setUser(name: String[50], gender: String[10]):
    assert gender == "male" or gender == "female" or gender == "other", "gender must be 'male', 'female' or 'other'"
    _gender_code: int128 = OTHER
    if gender == "male":
        _gender_code = MALE
    elif gender == "female":
        _gender_code = FEMALE
        _gender_code = OTHER
    self.user = User_obj({name: name, gender: _gender_code})

def getUser() -> (String[50], String[10]):
    _gender: String[10] = "other"
    if self.user.gender == MALE:
        _gender = "male"
    elif self.user.gender == FEMALE:
        _gender = "female"
        _gender = "other"
    return, _gender

Our smart contract stores a "User object" in the blockchain. It contains a name and a gender code. We have a setter-method setUser to set the name and the gender and a getter-method getUser to retrieve it.

As in the original blog post, we use web3 to deploy the smart contract and interact with it. The syntax is verion 5 of web3.

import json
from vyper.cli import vyper_compile
from web3 import Web3

# instance
w3 = Web3(Web3.HTTPProvider(""))

def deploy_contract(contract_interface):
    # Instantiate and deploy contract
    contract = w3.eth.contract(
        abi=contract_interface["abi"], bytecode=contract_interface["bytecode"]
    # Get transaction hash from deployed contract
    tx_hash = contract.constructor().transact({'from': w3.eth.accounts[1]})

    # Get tx receipt to get contract address
    tx_receipt = w3.eth.wait_for_transaction_receipt(tx_hash)
    return tx_receipt["contractAddress"]

def deploy_n_transact(file_path):
    output_formats = ["bytecode", "abi"]
    contract = vyper_compile.compile_files(file_path, output_formats)[file_path[0]]
    contract_address = deploy_contract(contract)
    return contract_address, contract["abi"]

# Compile and deploy Vyper source code
contract_address, abi = deploy_n_transact(['user.vy'])
data = {
    "abi": abi,
    "contract_address": contract_address
with open('data.json', 'w') as outfile:
    json.dump(data, outfile, indent=4, sort_keys=True)

Deploying the smart contract gives us the following result:

Deploy smart contract

Because we don't need the string library, we save a lot of gas. Yeah!

def check_gender(data):
    valid_list = ["male", "female", "other"]
    if data not in valid_list:
        raise ValidationError(f"Invalid gender. Valid choices are {valid_list}")

We add other as additional valid option for gender.

# api to set new user every api call
@app.route("/blockchain/user", methods=["POST"])
def transaction():
    w3.eth.defaultAccount = w3.eth.accounts[1]
    with open("data.json", "r") as f:
        datastore = json.load(f)
    abi = datastore["abi"]
    contract_address = datastore["contract_address"]

    # Create the contract instance with the newly-deployed address
    user = w3.eth.contract(
    body = request.get_json()
        result = UserSchema().load(body)
    except ValidationError as err:
        return jsonify(err.messages), 422

    tx_hash = user.functions.setUser(result["name"], result["gender"])
    tx_hash = tx_hash.transact()
    # Wait for transaction to be mined...

    user_data = user.functions.getUser().call()
    return jsonify({"data": user_data}), 200

With all our scripts in place we can spin off flask.  ../bin/flask run

and interact with the smart contract:

curl -H "Content-Type: application/json" --request POST -d '{"name":"Jane Doe","gender":"other"}' http://localhost:5000/blockchain/user
{"data":["Jane Doe","other"]}

The full code of the example you can find on Github.