# Form preview

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 `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 `'formtools'` to your `INSTALLED_APPS` setting.

     This will work if your `TEMPLATES` setting includes the `app_directories` template loader (which is the case by default).

     See the template loader docs for more.

   - Otherwise, determine the full filesystem path to the `formtools/templates` directory and add that directory to your `DIRS` option in the `TEMPLATES` setting.

2. Create a `FormPreview` subclass that overrides the `done()` method:

       from django.http import HttpResponseRedirect

       from 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 `HttpRequest` object and a dictionary of the form data after it has been validated and cleaned. It should return an `HttpResponseRedirect` that is the end result of the form being submitted.

3. Change your URLconf to point to an instance of your `FormPreview` subclass:

       from django import forms

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

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

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

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

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

## `FormPreview` classes

class `formtools.preview.``FormPreview`

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

## `FormPreview` templates

`FormPreview.``form_template`

`FormPreview.``preview_template`

By default, the form is rendered via the template `formtools/form.html`, and the preview page is rendered via the template `formtools/preview.html`.

These values can be overridden for a particular form preview by setting `preview_template` and `form_template` attributes on the FormPreview subclass. See `formtools/templates` for the default templates.

## Required methods

`FormPreview.``done`(request, cleaned_data)

  Does something with the `cleaned_data` data and then needs to return an `HttpResponseRedirect`, e.g. to a success page.

## Optional methods

`FormPreview.``get_auto_id`()

  Hook to override the `auto_id` kwarg for the form. Needed when rendering two form previews in the same template.

`FormPreview.``get_initial`(request)

  Takes a request argument and returns a dictionary to pass to the form’s `initial` kwarg when the form is being created from an HTTP get.

`FormPreview.``get_context`(request, form)

  Context for template rendering.

`FormPreview.``parse_params`(request, *args, **kwargs)

  Given captured args and kwargs from the URLconf, saves something in self.state and/or raises `Http404` if necessary.

  For example, this URLconf captures a user_id variable:

      (r'^contact/(?P<user_id>\d{1,6})/$', MyFormPreview(MyForm)),

  In this case, the kwargs variable in parse_params would be `{'user_id': 32}` for a request to `'/contact/32/'`. You can use that `user_id` to make sure it’s a valid user and/or save it for later, for use in `done()`.

`FormPreview.``process_preview`(request, form, context)

  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.

`FormPreview.``security_hash`(request, form)

  Calculates the security hash for the given `HttpRequest` and `Form` instances.

  Subclasses may want to take into account request-specific information, such as the IP address.

`FormPreview.``failed_hash`(request)

  Returns an `HttpResponse` in the case of an invalid security hash.