user.py

import sqlite3
from flask_restful import Resource, reqparse

class User:
    def __init__(self, _id, username, password):
        self.id = _id
        self.username = username
        self.password = password

    @classmethod
    def find_by_username(cls, username):
        connection =  sqlite3.connect('data.db')
        cursor = connection.cursor()

        query = "SELECT * FROM users WHERE username=?"
        result = cursor.execute(query, (username,))
        row = result.fetchone()
        if row:
            user = cls(*row) # *row instead of row[0], row[1], row[2]
        else:
            user = None

        connection.close()
        return user

    @classmethod
    def find_by_id(cls, _id):
        connection =  sqlite3.connect('data.db')
        cursor = connection.cursor()

        query = "SELECT * FROM users WHERE id=?"
        result = cursor.execute(query, (_id,))
        row = result.fetchone()
        if row:
            user = cls(*row) # *row instead of row[0], row[1], row[2]
        else:
            user = None

        connection.close()
        return user

class UserRegister(Resource):
    TABLE_NAME = 'users'

    parser = reqparse.RequestParser()
    parser.add_argument('username',
        type=str,
        required=True,
        help="This field cannot be blank!"
    )
    parser.add_argument('password',
        type=str,
        required=True,
        help="This field cannot be blank!"
    )
    def post(self):
        data = UserRegister.parser.parse_args()

        if User.find_by_username(data['username']):
            return {"message": "User with that username already exists."}, 400

        connection = sqlite3.connect('data.db')
        cursor = connection.cursor()

        query = "INSERT INTO {table} VALUES (NULL, ?, ?)".format(table=self.TABLE_NAME)
        cursor.execute(query, (data['username'], data['password']))

        connection.commit()
        connection.close()

        return {"message": "User created successfully."}, 201