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Changing Widget Defaults at Run Time

In order to modify a widget during run time one should use some special care. This document takes a SingleSelectField as an example to describe how some parameters can be changed in a widget at run time.

An archive containing a fully functional project is available for those who want to play directly with it :)

First quickstart a new project and call it “ssfdemo”, then edit ssfdemo/ssfdemo/ and add the following imports at the top in order to be able to use them during this example:

import random
import turbogears as tg
from turbogears.widgets import SingleSelectField, TableForm, WidgetsList

Then create a field list containing our SingleSelectField:

class FormFields(WidgetsList):
    single_select = SingleSelectField(
        label = _('Preferred Framework'),
        name = 'pref',
        options = [
            (0, 'The obscure one in Ruby'),
            (1, 'TurboGears'),
            (2, 'The one with that guitar player name'),
            (3, 'Pylons'),
            (4, ''),
            (5, 'Zope3'),
            (6, 'Zope2'),
            (7, 'Twisted+Nevow'),
            (8, 'Something super secret'),

Please note the name we have assigned to the SingleSelectField: pref. Now we will use this field list. Ok, this list may be overkill because there is only one element, but in real life your forms will generally contain more than one field.

Next, we just have to use this list of widgets into our container widget:

form = TableForm(
    name = 'form',
    fields = FormFields(),
    action = tg.url('post_handler'),
    submit_text = _('Vote')

We declared a form of name “form”, which uses our list of previously declared fields. Now amend the index method so it looks like this:

def index(self):
    pref = random.randint(0, 8)
    return dict(form=form, pref=pref}

The only thing we do here is to send our form and some contextual data, in that case a randomly choosen id. This data will be the one used to set the default value of our SingleSelectField.

But first we need to declare our post_handler because we made a reference to it in our form declaration. Here is a really simple form handler that will only print out the id that has been selected:

def post_handler(self, pref):
    return "You have chosen: %s" % pref

Note that the second argument of this method is the same as the name attribute of our SingleSelectField.

Now you can save the file and edit the ssfdemo/ssfdemo/templates/welcome.kid file. The easiest way to achieve our goal is to remove nearly everything from this welcome.kid file and make it look like this:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
<html xmlns=""
<meta content="text/html; charset=utf-8" http-equiv="Content-Type"
<title>Welcome to SingleSelectFieldDemo</title>
  <div id="getting_started">

The important part here is:


With this line we display the form but with amended values. It says: change the attribute named value on the sub-widget named pref by applying the content of pref to it. pref is the integer we randomly generated.

Save the file and test the result. Make sure to Shift-Reload the page in order to really update it.