The view function did not return a valid response [duplicate]

Answer 1

The error indicates that the function is not complete because it misses the return instruction. Any view function must return something. In your case you forgot this line which is used to indicate where is the template file and especially to render the form. Your code should look like this:

@app.route('/login', methods=['GET', 'POST'])
def login():
    if current_user.is_authenticated:
        return redirect(url_for('index'))
    form = LoginForm()
    if form.validate_on_submit():
        user = User.query.filter_by(username=form.username.data).first()
        if user is None or not user.check_password(form.password.data):
            flash('Invalid username or password')
            return redirect(url_for('login'))
        login_user(user, remember=form.remember_me.data)
        next_page = request.args.get('next')
        if not next_page or url_parse(next_page).netloc != '':
            next_page = url_for('index')
        return redirect(next_page)
    return render_template('login.html', form=form)

remarks the line: return render_template('login.html', form=form)