flask form.validate_on_submit() fails using wtforms

You call form.validate() everytime someone visits '/'. When the request is a GET, there is no form data causing validation to fail. You only want to try to validate the form when the request is a POST.

One way to do that is to check the request's method.

if request.method == 'POST':    if form.validate():        session['dataserver'] = ds = form.dataserver.data        return redirect(url_for('login'))    else:        flash('Failed validation')

Another common way to do this is with validate_on_submit. It handles checking for POSTs as well as validating the form.

if form.validate_on_submit():    session['dataserver'] = ds = form.dataserver.data    return redirect(url_for('login'))

Here you'd lose your ability to flash your 'validation failed' message. That may be acceptable, though, because you can check for form errors in your template.

{% if form.errors %}    failed validation{% endif %}

UPDATE

If you want to see the errors (which you may not), you can print them in the template instead of the generic "failed validation" message.

{% if form.errors %}    {{ form.errors }}{% endif %}