.. _ref-contrib-formtools-form-preview:

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

.. module:: django.contrib.formtools
    :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
       :meth:`~django.contrib.formtools.FormPreview.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.FormPreview` subclass that
       overrides the :meth:`~django.contrib.formtools.FormPreview.done()`
       method::

           from django.contrib.formtools.preview import FormPreview
           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.FormPreview` subclass::

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

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

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.FormPreview.preview_template` and
:attr:`~django.contrib.formtools.FormPreview.form_template` attributes on the
FormPreview subclass. See :file:`django/contrib/formtools/templates` for the
default templates.
