===========
Form wizard
===========

.. module:: django.contrib.formtools.wizard
    :synopsis: Splits forms across multiple Web pages.

Django comes with an optional "form wizard" application that splits
:doc:`forms </topics/forms/index>` across multiple Web pages. It maintains
state in hashed HTML :samp:`<input type="hidden">` fields so that the full
server-side processing can be delayed until the submission of the final form.

You might want to use this if you have a lengthy form that would be too
unwieldy for display on a single page. The first page might ask the user for
core information, the second page might ask for less important information,
etc.

The term "wizard," in this context, is `explained on Wikipedia`_.

.. _explained on Wikipedia: http://en.wikipedia.org/wiki/Wizard_%28software%29
.. _forms: ../forms/

How it works
============

Here's the basic workflow for how a user would use a wizard:

    1. The user visits the first page of the wizard, fills in the form and
       submits it.
    2. The server validates the data. If it's invalid, the form is displayed
       again, with error messages. If it's valid, the server calculates a
       secure hash of the data and presents the user with the next form,
       saving the validated data and hash in :samp:`<input type="hidden">`
       fields.
    3. Step 1 and 2 repeat, for every subsequent form in the wizard.
    4. Once the user has submitted all the forms and all the data has been
       validated, the wizard processes the data -- saving it to the database,
       sending an email, or whatever the application needs to do.

Usage
=====

This application handles as much machinery for you as possible. Generally, you
just have to do these things:

    1. Define a number of :class:`~django.forms.Form` classes -- one per wizard
       page.

    2. Create a :class:`FormWizard` class that specifies what to do once all of
       your forms have been submitted and validated. This also lets you
       override some of the wizard's behavior.

    3. Create some templates that render the forms. You can define a single,
       generic template to handle every one of the forms, or you can define a
       specific template for each form.

    4. Point your URLconf at your :class:`FormWizard` class.

Defining ``Form`` classes
=========================

The first step in creating a form wizard is to create the
:class:`~django.forms.Form` classes.  These should be standard
:class:`django.forms.Form` classes, covered in the :doc:`forms documentation
</topics/forms/index>`.  These classes can live anywhere in your codebase, but
convention is to put them in a file called :file:`forms.py` in your
application.

For example, let's write a "contact form" wizard, where the first page's form
collects the sender's email address and subject, and the second page collects
the message itself. Here's what the :file:`forms.py` might look like::

    from django import forms

    class ContactForm1(forms.Form):
        subject = forms.CharField(max_length=100)
        sender = forms.EmailField()

    class ContactForm2(forms.Form):
        message = forms.CharField(widget=forms.Textarea)

**Important limitation:** Because the wizard uses HTML hidden fields to store
data between pages, you may not include a :class:`~django.forms.FileField`
in any form except the last one.

Creating a ``FormWizard`` class
===============================

The next step is to create a
:class:`django.contrib.formtools.wizard.FormWizard` subclass.  As with your
:class:`~django.forms.Form` classes, this :class:`FormWizard` class can live
anywhere in your codebase, but convention is to put it in :file:`forms.py`.

The only requirement on this subclass is that it implement a
:meth:`~FormWizard.done()` method.

.. method:: FormWizard.done

    This method specifies what should happen when the data for *every* form is
    submitted and validated.  This method is passed two arguments:

        * ``request`` -- an :class:`~django.http.HttpRequest` object
        * ``form_list`` -- a list of :class:`~django.forms.Form` classes

In this simplistic example, rather than perform any database operation, the
method simply renders a template of the validated data::

    from django.shortcuts import render_to_response
    from django.contrib.formtools.wizard import FormWizard

    class ContactWizard(FormWizard):
        def done(self, request, form_list):
            return render_to_response('done.html', {
                'form_data': [form.cleaned_data for form in form_list],
            })

Note that this method will be called via ``POST``, so it really ought to be a
good Web citizen and redirect after processing the data. Here's another
example::

    from django.http import HttpResponseRedirect
    from django.contrib.formtools.wizard import FormWizard

    class ContactWizard(FormWizard):
        def done(self, request, form_list):
            do_something_with_the_form_data(form_list)
            return HttpResponseRedirect('/page-to-redirect-to-when-done/')

See the section `Advanced FormWizard methods`_ below to learn about more
:class:`FormWizard` hooks.

Creating templates for the forms
================================

Next, you'll need to create a template that renders the wizard's forms. By
default, every form uses a template called :file:`forms/wizard.html`. (You can
change this template name by overriding :meth:`~FormWizard.get_template()`,
which is documented below. This hook also allows you to use a different
template for each form.)

This template expects the following context:

    * ``step_field`` -- The name of the hidden field containing the step.
    * ``step0`` -- The current step (zero-based).
    * ``step`` -- The current step (one-based).
    * ``step_count`` -- The total number of steps.
    * ``form`` -- The :class:`~django.forms.Form` instance for the current step
      (either empty or with errors).
    * ``previous_fields`` -- A string representing every previous data field,
      plus hashes for completed forms, all in the form of hidden fields. Note
      that you'll need to run this through the :tfilter:`safe` template filter,
      to prevent auto-escaping, because it's raw HTML.

You can supply extra context to this template in two ways:

    * Set the :attr:`~FormWizard.extra_context` attribute on your
      :class:`FormWizard` subclass to a dictionary.

    * Pass a dictionary as a parameter named ``extra_context`` to your wizard's
      URL pattern in your URLconf.  See :ref:`hooking-wizard-into-urlconf`.

Here's a full example template:

.. code-block:: html+django

    {% extends "base.html" %}

    {% block content %}
    <p>Step {{ step }} of {{ step_count }}</p>
    <form action="." method="post">{% csrf_token %}
    <table>
    {{ form }}
    </table>
    <input type="hidden" name="{{ step_field }}" value="{{ step0 }}" />
    {{ previous_fields|safe }}
    <input type="submit">
    </form>
    {% endblock %}

Note that ``previous_fields``, ``step_field`` and ``step0`` are all required
for the wizard to work properly.

.. _hooking-wizard-into-urlconf:

Hooking the wizard into a URLconf
=================================

Finally, we need to specify which forms to use in the wizard, and then
deploy the new :class:`FormWizard` object a URL in ``urls.py``. The
wizard takes a list of your :class:`~django.forms.Form` objects as
arguments when you instantiate the Wizard::

    from django.conf.urls.defaults import *
    from testapp.forms import ContactForm1, ContactForm2, ContactWizard

    urlpatterns = patterns('',
        (r'^contact/$', ContactWizard([ContactForm1, ContactForm2])),
    )

Advanced ``FormWizard`` methods
===============================

.. class:: FormWizard

    Aside from the :meth:`~done()` method, :class:`FormWizard` offers a few
    advanced method hooks that let you customize how your wizard works.

    Some of these methods take an argument ``step``, which is a zero-based
    counter representing the current step of the wizard. (E.g., the first form
    is ``0`` and the second form is ``1``.)

.. method:: FormWizard.prefix_for_step

    Given the step, returns a form prefix to use.  By default, this simply uses
    the step itself. For more, see the :ref:`form prefix documentation
    <form-prefix>`.

    Default implementation::

        def prefix_for_step(self, step):
            return str(step)

.. method:: FormWizard.render_hash_failure

    Renders a template if the hash check fails. It's rare that you'd need to
    override this.

    Default implementation::

        def render_hash_failure(self, request, step):
            return self.render(self.get_form(step), request, step,
                context={'wizard_error':
                             'We apologize, but your form has expired. Please'
                             ' continue filling out the form from this page.'})

.. method:: FormWizard.security_hash

    Calculates the security hash for the given request object and
    :class:`~django.forms.Form` instance.

    By default, this generates a SHA1 HMAC using your form data and your
    :setting:`SECRET_KEY` setting. It's rare that somebody would need to
    override this.

    Example::

        def security_hash(self, request, form):
            return my_hash_function(request, form)

.. method:: FormWizard.parse_params

    A hook for saving state from the request object and ``args`` / ``kwargs``
    that were captured from the URL by your URLconf.

    By default, this does nothing.

    Example::

        def parse_params(self, request, *args, **kwargs):
            self.my_state = args[0]

.. method:: FormWizard.get_template

    Returns the name of the template that should be used for the given step.

    By default, this returns :file:`'forms/wizard.html'`, regardless of step.

    Example::

        def get_template(self, step):
            return 'myapp/wizard_%s.html' % step

    If :meth:`~FormWizard.get_template` returns a list of strings, then the
    wizard will use the template system's
    :func:`~django.template.loader.select_template` function.
    This means the system will use the first template that exists on the
    filesystem. For example::

        def get_template(self, step):
            return ['myapp/wizard_%s.html' % step, 'myapp/wizard.html']

.. method:: FormWizard.render_template

    Renders the template for the given step, returning an
    :class:`~django.http.HttpResponse` object.

    Override this method if you want to add a custom context, return a
    different MIME type, etc. If you only need to override the template name,
    use :meth:`~FormWizard.get_template` instead.

    The template will be rendered with the context documented in the
    "Creating templates for the forms" section above.

.. method:: FormWizard.process_step

    Hook for modifying the wizard's internal state, given a fully validated
    :class:`~django.forms.Form` object. The Form is guaranteed to have clean,
    valid data.

    This method should *not* modify any of that data. Rather, it might want to
    set ``self.extra_context`` or dynamically alter ``self.form_list``, based
    on previously submitted forms.

    Note that this method is called every time a page is rendered for *all*
    submitted steps.

    The function signature::

        def process_step(self, request, form, step):
            # ...