Question

I use WTForms and Flask with Flask-WTF extension

My form looks like:

class CommentForm(Form):
    body = TextAreaField('Body', [validators.Length(min=4, max=300)])
    entity_id = HiddenField('Entity ID', [validators.required()])

Jinja2 Template:

 <form method="POST" action="{{ request.url }}#comment-question" id="comment-question">
     <div>{{ comment_form.body }} <button type="submit">Submit</button></div>
     {{ comment_form.entity_id(value=question.id) }}
     {{ comment_form.hidden_tag() }}
 </form>

Rendered form:

<form method="POST" action="http://localhost:5000/answers/1/question-0#comment-question" id="comment-question">
  <div><textarea id="body" name="body"></textarea> <button type="submit">Submit</button></div>
  <input id="entity_id" name="entity_id" type="hidden" value="1">
  <div style="display:none;"><input id="csrf_token" name="csrf_token" type="hidden" value="20120507081937##ee73cc3cfc053266fef78b48cc645cbf90e8fba6"><input id="entity_id" name="entity_id" type="hidden" value=""></div>
</form>

Is it possible to prevent the double form submit on the browser refresh button click without changing the form “action” and doing redirects?

Answer 1

I don’t have too much experience using WTForms or Flask, but Django class-based views prevent double-posts by redirecting after a POST, so I had assumed performing a redirect is the way to go for this sort of thing.

One alternative is to generate a unique token and attach it to your form parameters (much like a CSRF token). Cache this value and check against it on form submission. A rather primitive example for Django can be found here.

Edit: Sample code

Although I would really just go with performing a redirect after a successful form submission, here’s an example of generating a form token which borrows heavily from this Flask snippet on CSRF protection:

# yourapp/views/filters.py

import random
from string import ascii_letters, digits

from flask import request, session, redirect
from yourapp import app


def generate_form_token():
    """Sets a token to prevent double posts."""
    if '_form_token' not in session:
        form_token = \
            ''.join([random.choice(ascii_letters+digits) for i in range(32)])
        session['_form_token'] = form_token
    return session['_form_token']


@app.before_request
def check_form_token():
    """Checks for a valid form token in POST requests."""
    if request.method == 'POST':
        token = session.pop('_form_token', None)
        if not token or token != request.form.get('_form_token'):
            redirect(request.url)


app.jinja_env.globals['form_token'] = generate_form_token

And in your template:

<!-- Again, I've never used WTForms so I'm not sure if this would change when using that app. -->
<input type='hidden' name='_form_token' value='{{ form_token() }}' />

Note that using the CSRF protection method in the snippet also accomplishes pretty much the same effect (although the above code performs a redirect, while the snippet returns a 403).

But this really begs the question–if you’re performing a redirect on an invalid token, why not get rid of all this complexity and just redirect on successful form submission?