Flask-html : Best way to validate form input

Question

Hello I'm trying to create a web registration form with the flask module. What is the easiest way/module to check if a username contains a certain amount of characters, has numbers, and uppercase letters and how do I loop a form input until a valid username in this case is entered?

@app.route('/register', methods=['POST', 'GET'])
def register():
    if request.method == "POST":
        username = request.form["username"]
        password = request.form["password"]
        error = None
        if not username:
            error = "Username is required."
        elif not password:
            error = "Password is required."
            flash(error)
        if error is None:
            with open('data.txt', 'a', encoding='utf8') as file:
                encoded_password = password.encode()
                hash_password = hashlib.sha256(encoded_password).hexdigest()
                file.write(username + ' ' + hash_password + '\n')
            return redirect(url_for("login"))
    return render_template('register.html')

Answer 1

call another function like this (just an example only worrying about the username) and expand the logic as you see fit - determine if they should stay on the registration page based on whether process_registration is True

def verify_user_registration_credentials(username, password):
    if not username:
        flash_message = "Please enter a username"
        process_registration = False
    elif len(username) <= 5:
        flash_message = "Please enter a username greater than 5 characters"
        process_registration = False
    else: 
         for character in username:
             if character.isdigit() or character.isupper():
                 break
                 process_registration = True # assuming you want them to have either a number or an upper case letter in their username