Lincoln Loop

Better Use of Newforms

The newforms library is a huge time-saver, but when I first started using it, I still found myself writing tedious repetitve code to get it to function how I wanted. While I could get away with it on smaller sites, I recently built a site with some big forms on it and decided to improve my process.

HTML Rendering

First off, {{ form }} or {{ form.as_p }}, rarely cut it in real world apps. We need to be able to customize our forms to improve the layout or add extra information. I started using inclusion tags to render the form fields and labels. Here is my trivial inclusion tag:

@register.inclusion_tag('_display_field.html')
def display_field(field, alt_label=''):
    """
    Print HTML for a newform field. 
    Optionally, a label can be supplied that overrides the default label generated for the form.
    
    Example:
    {% display_field form.my_field "My New Label" %}
    """

    if alt_label:
        field.label = alt_label
    return { 'field': field }

And the accompanying HTML:

{% if field.errors %}
<p class="error">{{ field.errors|join:"<br />" }}</p>
{% endif %}
<label for="{{ field.auto_id }}">
    {{ field.label }}
    {% if field.field.required %}<br />
<span class="required">required</span>
    {% endif %}
</label> 
{{ field }} <br />

This should all be pretty straight forward. The only bit of trickery here is where newforms hides the required value for a field, field.field.required.

This little bit of code changed some unmanagebly large forms into an HTML template that looks like this:

{% display_field form.first_name %}
{% display_field form.last_name %}
{% display_field form.email %}

Now that’s a form you can live with!

update 3/25: There has been some chat on the developer mailing list about fixing some of this inside Django itself.

Form Handling

This is a simple little trick, but it will go a long way towards keeping your code clean and DRY. If you often (or always) perform the same task when processing a form, get that code out of your views. Instead, create a method to handle it inside the form itself. This keeps your views easy to read and makes it easy to edit if you ever make changes to the form itself.

Here’s a quick example of creating a new user:

class UserForm(forms.Form):
    first_name = forms.CharField()
    last_name = forms.CharField()
    email = forms.EmailField()
    username = forms.CharField()
    password = forms.CharField(widget=forms.PasswordInput(render_value=False))
        
    def save(self):
        new_user = User.objects.create_user(username=self.cleaned_data['username'],
                                            email=self.cleaned_data['email'],
                                            password=self.cleaned_data['password'])
        new_user.first_name = self.cleaned_data['first_name']
        new_user.last_name = self.cleaned_data['last_name']
        new_user.is_active = False
        new_user.save()
        return new_user

NB: For the sake of brevity, this leaves out some important error checking. Don’t use this code in production.

Now your view code is dead simple:

form = UserForm(request.POST)
if form.is_valid():
    user = form.save()

Hope you found this useful. More newforms shortcuts in the coming weeks.

Comments

• March 25, 2008 at 2:46 p.m. #
  Jökull piped up:

  I used to put all my .save() in my forms. However over time I found myself going back and forth between view.py and forms.py because so much business logic ends up in the .save(). Just a heads up. Something to think about if you like to keep your business logic in one place.

  —

  One tool I’ve used a lot is an update_object helper function. If my cleaned_dict matches the object properties I can do this: obj = update_object(Object, **cleaned_dict) and then obj.save().

  def update_object(o, **attrs):
      for k, v in attrs.iteritems():
          setattr(o, k, v)
      return o
  

• March 25, 2008 at 4:28 p.m. #
  pytechd piped up:

  I have a quick little code library that lets me do similar, but is more controlled in Python since I rarely adjust the actual form output methods. I don’t like the overhead of WTForm (and it wasn’t around when I started my project). I can e-mail it and should have it on djangosnippets in a few days.

  {% form %} {% field form.first_name span=6 %} {% field form.last_name span=6 %}
  {% endform %}

  It’s table based. CSS forms are nice, but are havoc when your application is based on a LOT of complicated forms. The little template tag {% form %} sets up a table with appropriate columns (12 — lets you do 2, 3, and 4 columns easily).

  Very similar ideas. :)

• April 3, 2008 at 4:06 a.m. #
  emes mentioned:

  There’s already a tool for that in Django, called newforms.ModelForm. Does the same in save() automatically. The only part left for you is data validation:

  http://www.djangoproject.com/documentation/modelforms/

• April 3, 2008 at 8:23 a.m. #
  Peter Baumgartner added:

  @emes, Yes, ModelForm works great when your form translates very closely to a model, but when building real world apps, that isn’t always going to be the case. Take a look at my FormMail clone for an example of this technique that uses a different approach.

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