============
Form preview
============

.. module:: django.contrib.formtools.preview
    :synopsis: Displays an HTML form, forces a preview, then does something
               with the submission.

Django comes with an optional "form preview" application that helps automate
the following workflow:

"Display an HTML form, force a preview, then do something with the submission."

To force a preview of a form submission, all you have to do is write a short
Python class.

Overview
=========

Given a :class:`django.forms.Form` subclass that you define, this
application takes care of the following workflow:

1. Displays the form as HTML on a Web page.
2. Validates the form data when it's submitted via POST.
   a. If it's valid, displays a preview page.
   b. If it's not valid, redisplays the form with error messages.
3. When the "confirmation" form is submitted from the preview page, calls
   a hook that you define -- a ``done()`` method that gets passed the valid
   data.

The framework enforces the required preview by passing a shared-secret hash to
the preview page via hidden form fields. If somebody tweaks the form parameters
on the preview page, the form submission will fail the hash-comparison test.

How to use ``FormPreview``
==========================

1. Point Django at the default FormPreview templates. There are two ways to
   do this:

   * Add ``'django.contrib.formtools'`` to your
     :setting:`INSTALLED_APPS` setting. This will work if your
     :setting:`TEMPLATE_LOADERS` setting includes the
     ``app_directories`` template loader (which is the case by
     default). See the :ref:`template loader docs <template-loaders>`
     for more.

   * Otherwise, determine the full filesystem path to the
     :file:`django/contrib/formtools/templates` directory, and add that
     directory to your :setting:`TEMPLATE_DIRS` setting.

2. Create a :class:`~django.contrib.formtools.preview.FormPreview` subclass that
   overrides the ``done()`` method::

       from django.contrib.formtools.preview import FormPreview
       from django.http import HttpResponseRedirect
       from myapp.models import SomeModel

       class SomeModelFormPreview(FormPreview):

           def done(self, request, cleaned_data):
               # Do something with the cleaned_data, then redirect
               # to a "success" page.
               return HttpResponseRedirect('/form/success')

   This method takes an :class:`~django.http.HttpRequest` object and a
   dictionary of the form data after it has been validated and cleaned.
   It should return an :class:`~django.http.HttpResponseRedirect` that
   is the end result of the form being submitted.

3. Change your URLconf to point to an instance of your
   :class:`~django.contrib.formtools.preview.FormPreview` subclass::

       from myapp.preview import SomeModelFormPreview
       from myapp.forms import SomeModelForm
       from django import forms

   ...and add the following line to the appropriate model in your URLconf::

       (r'^post/$', SomeModelFormPreview(SomeModelForm)),

   where ``SomeModelForm`` is a Form or ModelForm class for the model.

4. Run the Django server and visit :file:`/post/` in your browser.

``FormPreview`` classes
=======================

.. class:: FormPreview

A :class:`~django.contrib.formtools.preview.FormPreview` class is a simple Python class
that represents the preview workflow.
:class:`~django.contrib.formtools.preview.FormPreview` classes must subclass
``django.contrib.formtools.preview.FormPreview`` and override the ``done()``
method. They can live anywhere in your codebase.

``FormPreview`` templates
=========================

.. attribute:: FormPreview.form_template
.. attribute:: FormPreview.preview_template

By default, the form is rendered via the template :file:`formtools/form.html`,
and the preview page is rendered via the template :file:`formtools/preview.html`.
These values can be overridden for a particular form preview by setting
:attr:`~django.contrib.formtools.preview.FormPreview.preview_template` and
:attr:`~django.contrib.formtools.preview.FormPreview.form_template` attributes on the
FormPreview subclass. See :file:`django/contrib/formtools/templates` for the
default templates.

Advanced ``FormPreview`` methods
================================

.. method:: FormPreview.process_preview()

    Given a validated form, performs any extra processing before displaying the
    preview page, and saves any extra data in context.

    By default, this method is empty.  It is called after the form is validated,
    but before the context is modified with hash information and rendered.