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 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 INSTALLED_APPS setting. This will work if your TEMPLATE_LOADERS 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 django/contrib/formtools/templates directory, and add that directory to your TEMPLATE_DIRS setting.

  2. Create a FormPreview subclass that overrides the 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 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 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 /post/ in your browser.

FormPreview classes

class FormPreview

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

FormPreview templates

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 django/contrib/formtools/templates for the default templates.

Advanced FormPreview methods

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.

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