Source

wtforms / docs / ext.rst

Extensions

WTForms ships with a number of extensions that make it easier to work with other frameworks and libraries, such as Django.

Appengine

WTForms includes support for AppEngine fields as well as auto-form generation from models.

Note

WTForms supports both appengine.ext.db and appengine.ext.ndb style models now, and there is some overlap between them. For the near future, we will continue to support both, but at some point WTForms will go to only supporting AppEngine for python 2.7 and drop support for ext.db models as well.

Model Forms

See the module docstring for examples on how to use :func:`model_form`.

Datastore-backed Fields

NDB

WTForms now includes support for NDB models and can support mapping the relationship fields as well as generating forms from models.

Dateutil

For better date-time parsing using the python-dateutil package, :mod:`wtforms.ext.dateutil` provides a set of fields to use to accept a wider range of date input.

Django

This extension provides templatetags to make it easier to work with Django templates and WTForms' html attribute rendering. It also provides a generator for automatically creating forms based on Django ORM models.

Templatetags

Django templates does not allow arbitrarily calling functions with parameters, making it impossible to use the html attribute rendering feature of WTForms. To alleviate this, we provide a templatetag.

Adding :mod:`wtforms.ext.django` to your INSTALLED_APPS will make the wtforms template library available to your application. With this you can pass extra attributes to form fields similar to the usage in jinja:

{% load wtforms %}

{% form_field form.username class="big_text" onclick="do_something()" %}

Note By default, using the {{ form.field }} syntax in django models will be auto-escaped. To avoid this happening, use Django's {% autoescape off %} block tag or use WTForms' form_field template tag.

Model forms

ORM-backed fields

While linking data to most fields is fairly easy, making drop-down select lists using django ORM data can be quite repetitive. To this end, we have added some helpful tools to use the django ORM along with wtforms.

SQLAlchemy

This extension provides SelectField integration with SQLAlchemy ORM models, similar to those in the Django extension.

ORM-backed fields

These fields are provided to make it easier to use data from ORM objects in your forms.

def enabled_categories():
    return Category.query.filter_by(enabled=True)

class BlogPostEdit(Form):
    title    = StringField()
    blog     = QuerySelectField(get_label='title')
    category = QuerySelectField(query_factory=enabled_categories, allow_blank=True)

def edit_blog_post(request, id):
    post = Post.query.get(id)
    form = ArticleEdit(obj=post)
    # Since we didn't provide a query_factory for the 'blog' field, we need
    # to set a dynamic one in the view.
    form.blog.query = Blog.query.filter(Blog.author == request.user).order_by(Blog.name)

CSRF

The CSRF package includes tools that help you implement checking against cross-site request forgery ("csrf"). Due to the large number of variations on approaches people take to CSRF (and the fact that many make compromises) the base implementation allows you to plug in a number of CSRF validation approaches.

CSRF implementations are made by subclassing :class:`~wtforms.ext.csrf.form.SecureForm`. For utility, we have provided one possible CSRF implementation in the package that can be used with many frameworks for session-based hash secure keying, :class:`~wtforms.ext.csrf.session.SessionSecureForm`.

All CSRF implementations hinge around creating a special token, which is put in a hidden field on the form named 'csrf_token', which must be rendered in your template to be passed from the browser back to your view. There are many different methods of generating this token, but they are usually the result of a cryptographic hash function against some data which would be hard to forge.

Creating your own CSRF implementation

Here we will sketch out a simple theoretical CSRF implementation which generates a hash token based on the user's IP.

Note This is a simplistic example meant to illustrate creating a CSRF implementation. This isn't recommended to be used in production because the token is deterministic and non-changing per-IP, which means this isn't the most secure implementation of CSRF.

First, let's create our SecureForm base class:

from wtforms.ext.csrf import SecureForm
from hashlib import md5

SECRET_KEY = '1234567890'

class IPSecureForm(SecureForm):
    """
    Generate a CSRF token based on the user's IP. I am probably not very
    secure, so don't use me.
    """

    def generate_csrf_token(self, csrf_context):
        # csrf_context is passed transparently from the form constructor,
        # in this case it's the IP address of the user
        token = md5(SECRET_KEY + csrf_context).hexdigest()
        return token

    def validate_csrf_token(self, field):
        if field.data != field.current_token:
            raise ValueError('Invalid CSRF')

Now that we have this taken care of, let's write a simple form and view which would implement this:

class RegistrationForm(IPSecureForm):
    name = StringField('Your Name')
    email = StringField('Email', [validators.email()])

def register(request):
    form = RegistrationForm(request.POST, csrf_context=request.ip)

    if request.method == 'POST' and form.validate():
        pass # We're all good, create a user or whatever it is you do
    elif form.csrf_token.errors:
        pass # If we're here we suspect the user of cross-site request forgery
    else:
        pass # Any other errors

    return render('register.html', form=form)

And finally, a simple template:

<form action="register" method="POST">
    {{ form.csrf_token }}
    <p>{{ form.name.label }}: {{ form.name }}</p>
    <p>{{ form.email.label }}: {{ form.email }}</p>
    <input type="submit" value="Register">
</form>

Please note that implementing CSRF detection is not fool-proof, and even with the best CSRF protection implementation, it's possible for requests to be forged by expert attackers. However, a good CSRF protection would make it infeasible for someone from an external site to hijack a form submission from another user and perform actions as them without additional a priori knowledge.

In addition, it's important to understand that very often, the more strict the CSRF protection, the higher the chance of false positives occurring (ie, legitimate users getting blocked by your CSRF protection) and choosing a CSRF implementation is actually a matter of compromise. We will attempt to provide a handful of usable reference algorithms built in to this library in the future, to allow that choice to be easy.

Some tips on criteria people often examine when evaluating CSRF implementations:

  • Reproducability If a token is based on attributes about the user, it gains the advantage that one does not need secondary storage in which to store the value between requests. However, if the same attributes can be reproduced by an attacker, then the attacker can potentially forge this information.
  • Reusability. It might be desired to make a completely different token every use, and disallow users from re-using past tokens. This is an extremely powerful protection, but can have consequences on if the user uses the back button (or in some cases runs forms simultaneously in multiple browser tabs) and submits an old token, or otherwise. A possible compromise is to allow reusability in a time window (more on that later).
  • Time Ranges Many CSRF approaches use time-based expiry to make sure that a token cannot be (re)used beyond a certain point. Care must be taken in choosing the time criteria for this to not lock out legitimate users. For example, if a user might walk away while filling out a long-ish form, or to go look for their credit card, the time for expiry should take that into consideration to provide a balance between security and limiting user inconvenience.
  • Requirements Some CSRF-prevention methods require the use of browser cookies, and some even require client-side scripting support. The webmaster implementing the CSRF needs to consider that such requirements (though effective) may lock certain legitimate users out, and make this determination whether it is a good idea to use. For example, for a site already using cookies for login, adding another for CSRF isn't as big of a deal, but for other sites it may not be feasible.

Session-based CSRF implementation

Usage

First, create a SessionSecureForm subclass that you can use as your base class for any forms you want CSRF support for:

from wtforms.ext.csrf.session import SessionSecureForm

class MyBaseForm(SessionSecureForm):
    SECRET_KEY = 'EPj00jpfj8Gx1SjnyLxwBBSQfnQ9DJYe0Ym'
    TIME_LIMIT = timedelta(minutes=20)

Now incorporate it into any form/view by further subclassing:

class Registration(MyBaseForm):
    name = StringField()

def view(request):
    form = Registration(request.POST, csrf_context=request.session)
    # rest of view here

Note that request.session is passed as the csrf_context= parameter, this is so that the CSRF token can be stored in your session for comparison on a later request.

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