buy function

@app.route("/buy", methods=["GET", "POST"])
@login_required
def buy():
    if request.method =="POST":
        # use lookup to verify the stock
        stock = lookup(request.form.get("stock"))
        if not stock:
            return apology("The stock you entered does not exist.", 403)
        # check if shares are valid integers
        try:
            shares = int(request.form.get("shares"))
            if shares <= 0:
                return apology("Enter a positive number", 403)
        except ValueError:
            return apology("Enter a positive number", 403)
        # check if user has enough cash
        # db.execute returns a list of dictionaries which needs to be accessed and then comapared
        cash = db.execute("SELECT * FROM users WHERE id = :id", \
               id = session["user_id"])
        if cash[0]["cash"] < (shares * float(stock["price"])) or not cash:
            return apology("You don't have enough money")
        else:
            # add to database in portfolio
            db.execute("INSERT INTO portfolio (user_id, symbol, shares, price) VALUES(:user_id, :symbol, :shares, :price)", \
                       user_id = session["user_id"], symbol = request.form.get("stock"), shares = request.form.get("shares"), price = shares*float(stock["price"]))

            # update cash in users table UPDATE users SET cash = cash - 50 WHERE id = :id
            db.execute("UPDATE users SET cash = cash - :purchase WHERE id = :id", \
            id = session["user_id"], purchase = shares*float(stock["price"]))

            # redirect to portfolio page
            return redirect("/")


    else:
        return render_template("buy.html")