@app.route("/register", methods=["GET", "POST"])
def register():
    """Register user"""

    # User reached route via POST (as by submitting a form via POST)
    if request.method == "POST":

        # Stored POST data to variables
        username = request.form.get('username')
        password = request.form.get('password')
        confirmation = request.form.get('confirmation')

        # Ensure username was submitted
        if not username:
           return apology("must provide username", 403)

        # Ensure password and confirmation were both submitted
        elif not password:
            return apology("must provide password in both fields", 403)

        # Ensure passwords match
        elif confirmation != password:
            return apology("passwords do not match", 403)

        elif db.execute('SELECT id FROM users WHERE username = ?',
        (username,)).fetchone()  is not None:
            return apology("Username already exists", 403)

elif db.execute('SELECT id FROM users WHERE username = ?',
(username,)).fetchone()  is not None:
TypeError: execute() takes 2 positional arguments but 3 were given

from cs50 import SQL; and
db = SQL("sqlite:///finance.db")

import sqlite3; and
db = sqlite3.connect('finance.db')

elif db.execute('SELECT id FROM users WHERE username = ?',
    (username,)).fetchone()  is not None:
        return apology("Username already exists", 403)