diff -r 261778de26ff -r 620f9b141567 thirdparty/google_appengine/lib/django/docs/forms.txt --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/thirdparty/google_appengine/lib/django/docs/forms.txt Tue Aug 26 21:49:54 2008 +0000 @@ -0,0 +1,695 @@ +=============================== +Forms, fields, and manipulators +=============================== + +Forwards-compatibility note +=========================== + +The legacy forms/manipulators system described in this document is going to be +replaced in the next Django release. If you're starting from scratch, we +strongly encourage you not to waste your time learning this. Instead, learn and +use the django.newforms system, which we have begun to document in the +`newforms documentation`_. + +If you have legacy form/manipulator code, read the "Migration plan" section in +that document to understand how we're making the switch. + +.. _newforms documentation: ../newforms/ + +Introduction +============ + +Once you've got a chance to play with Django's admin interface, you'll probably +wonder if the fantastic form validation framework it uses is available to user +code. It is, and this document explains how the framework works. + +We'll take a top-down approach to examining Django's form validation framework, +because much of the time you won't need to use the lower-level APIs. Throughout +this document, we'll be working with the following model, a "place" object:: + + from django.db import models + + PLACE_TYPES = ( + (1, 'Bar'), + (2, 'Restaurant'), + (3, 'Movie Theater'), + (4, 'Secret Hideout'), + ) + + class Place(models.Model): + name = models.CharField(maxlength=100) + address = models.CharField(maxlength=100, blank=True) + city = models.CharField(maxlength=50, blank=True) + state = models.USStateField() + zip_code = models.CharField(maxlength=5, blank=True) + place_type = models.IntegerField(choices=PLACE_TYPES) + + class Admin: + pass + + def __str__(self): + return self.name + +Defining the above class is enough to create an admin interface to a ``Place``, +but what if you want to allow public users to submit places? + +Automatic Manipulators +====================== + +The highest-level interface for object creation and modification is the +**automatic Manipulator** framework. An automatic manipulator is a utility +class tied to a given model that "knows" how to create or modify instances of +that model and how to validate data for the object. Automatic Manipulators come +in two flavors: ``AddManipulators`` and ``ChangeManipulators``. Functionally +they are quite similar, but the former knows how to create new instances of the +model, while the latter modifies existing instances. Both types of classes are +automatically created when you define a new class:: + + >>> from mysite.myapp.models import Place + >>> Place.AddManipulator + + >>> Place.ChangeManipulator + + +Using the ``AddManipulator`` +---------------------------- + +We'll start with the ``AddManipulator``. Here's a very simple view that takes +POSTed data from the browser and creates a new ``Place`` object:: + + from django.shortcuts import render_to_response + from django.http import Http404, HttpResponse, HttpResponseRedirect + from django import forms + from mysite.myapp.models import Place + + def naive_create_place(request): + """A naive approach to creating places; don't actually use this!""" + # Create the AddManipulator. + manipulator = Place.AddManipulator() + + # Make a copy of the POSTed data so that do_html2python can + # modify it in place (request.POST is immutable). + new_data = request.POST.copy() + + # Convert the request data (which will all be strings) into the + # appropriate Python types for those fields. + manipulator.do_html2python(new_data) + + # Save the new object. + new_place = manipulator.save(new_data) + + # It worked! + return HttpResponse("Place created: %s" % new_place) + +The ``naive_create_place`` example works, but as you probably can tell, this +view has a number of problems: + + * No validation of any sort is performed. If, for example, the ``name`` field + isn't given in ``request.POST``, the save step will cause a database error + because that field is required. Ugly. + + * Even if you *do* perform validation, there's still no way to give that + information to the user in any sort of useful way. + + * You'll have to separately create a form (and view) that submits to this + page, which is a pain and is redundant. + +Let's dodge these problems momentarily to take a look at how you could create a +view with a form that submits to this flawed creation view:: + + def naive_create_place_form(request): + """Simplistic place form view; don't actually use anything like this!""" + # Create a FormWrapper object that the template can use. Ignore + # the last two arguments to FormWrapper for now. + form = forms.FormWrapper(Place.AddManipulator(), {}, {}) + return render_to_response('places/naive_create_form.html', {'form': form}) + +(This view, as well as all the following ones, has the same imports as in the +first example above.) + +The ``forms.FormWrapper`` object is a wrapper that templates can +easily deal with to create forms. Here's the ``naive_create_form.html`` +template:: + + {% extends "base.html" %} + + {% block content %} +

Create a place:

+ +
+

{{ form.name }}

+

{{ form.address }}

+

{{ form.city }}

+

{{ form.state }}

+

{{ form.zip_code }}

+

{{ form.place_type }}

+ +
+ {% endblock %} + +Before we get back to the problems with these naive set of views, let's go over +some salient points of the above template: + + * Field "widgets" are handled for you: ``{{ form.field }}`` automatically + creates the "right" type of widget for the form, as you can see with the + ``place_type`` field above. + + * There isn't a way just to spit out the form. You'll still need to define + how the form gets laid out. This is a feature: Every form should be + designed differently. Django doesn't force you into any type of mold. + If you must use tables, use tables. If you're a semantic purist, you can + probably find better HTML than in the above template. + + * To avoid name conflicts, the ``id`` values of form elements take the + form "id_*fieldname*". + +By creating a creation form we've solved problem number 3 above, but we still +don't have any validation. Let's revise the validation issue by writing a new +creation view that takes validation into account:: + + def create_place_with_validation(request): + manipulator = Place.AddManipulator() + new_data = request.POST.copy() + + # Check for validation errors + errors = manipulator.get_validation_errors(new_data) + manipulator.do_html2python(new_data) + if errors: + return render_to_response('places/errors.html', {'errors': errors}) + else: + new_place = manipulator.save(new_data) + return HttpResponse("Place created: %s" % new_place) + +In this new version, errors will be found -- ``manipulator.get_validation_errors`` +handles all the validation for you -- and those errors can be nicely presented +on an error page (templated, of course):: + + {% extends "base.html" %} + + {% block content %} + +

Please go back and correct the following error{{ errors|pluralize }}:

+ + + {% endblock %} + +Still, this has its own problems: + + * There's still the issue of creating a separate (redundant) view for the + submission form. + + * Errors, though nicely presented, are on a separate page, so the user will + have to use the "back" button to fix errors. That's ridiculous and unusable. + +The best way to deal with these issues is to collapse the two views -- the form +and the submission -- into a single view. This view will be responsible for +creating the form, validating POSTed data, and creating the new object (if the +data is valid). An added bonus of this approach is that errors and the form will +both be available on the same page, so errors with fields can be presented in +context. + +.. admonition:: Philosophy: + + Finally, for the HTTP purists in the audience (and the authorship), this + nicely matches the "true" meanings of HTTP GET and HTTP POST: GET fetches + the form, and POST creates the new object. + +Below is the finished view:: + + def create_place(request): + manipulator = Place.AddManipulator() + + if request.method == 'POST': + # If data was POSTed, we're trying to create a new Place. + new_data = request.POST.copy() + + # Check for errors. + errors = manipulator.get_validation_errors(new_data) + manipulator.do_html2python(new_data) + + if not errors: + # No errors. This means we can save the data! + new_place = manipulator.save(new_data) + + # Redirect to the object's "edit" page. Always use a redirect + # after POST data, so that reloads don't accidently create + # duplicate entires, and so users don't see the confusing + # "Repost POST data?" alert box in their browsers. + return HttpResponseRedirect("/places/edit/%i/" % new_place.id) + else: + # No POST, so we want a brand new form without any data or errors. + errors = new_data = {} + + # Create the FormWrapper, template, context, response. + form = forms.FormWrapper(manipulator, new_data, errors) + return render_to_response('places/create_form.html', {'form': form}) + +and here's the ``create_form`` template:: + + {% extends "base.html" %} + + {% block content %} +

Create a place:

+ + {% if form.has_errors %} +

Please correct the following error{{ form.error_dict|pluralize }}:

+ {% endif %} + +
+

+ {{ form.name }} + {% if form.name.errors %}*** {{ form.name.errors|join:", " }}{% endif %} +

+

+ {{ form.address }} + {% if form.address.errors %}*** {{ form.address.errors|join:", " }}{% endif %} +

+

+ {{ form.city }} + {% if form.city.errors %}*** {{ form.city.errors|join:", " }}{% endif %} +

+

+ {{ form.state }} + {% if form.state.errors %}*** {{ form.state.errors|join:", " }}{% endif %} +

+

+ {{ form.zip_code }} + {% if form.zip_code.errors %}*** {{ form.zip_code.errors|join:", " }}{% endif %} +

+

+ {{ form.place_type }} + {% if form.place_type.errors %}*** {{ form.place_type.errors|join:", " }}{% endif %} +

+ +
+ {% endblock %} + +The second two arguments to ``FormWrapper`` (``new_data`` and ``errors``) +deserve some mention. + +The first is any "default" data to be used as values for the fields. Pulling +the data from ``request.POST``, as is done above, makes sure that if there are +errors, the values the user put in aren't lost. If you try the above example, +you'll see this in action. + +The second argument is the error list retrieved from +``manipulator.get_validation_errors``. When passed into the ``FormWrapper``, +this gives each field an ``errors`` item (which is a list of error messages +associated with the field) as well as a ``html_error_list`` item, which is a +``