Widgets are classes whose purpose are to render a field to its usable representation, usually XHTML. When a field is called, the default behaviour is to delegate the rendering to its widget. This abstraction is provided so that widgets can easily be created to customize the rendering of existing fields.
Note All built-in widgets will return upon rendering a "HTML-safe" unicode string subclass that many templating frameworks (Jinja2, Mako, Genshi) will recognize as not needing to be auto-escaped.
Widgets, much like validators, provide a simple callable contract. Widgets can take customization arguments through a constructor if needed as well. When the field is called or printed, it will call the widget with itself as the first argument and then any additional arguments passed to its caller as keywords. Passing the field is done so that one instance of a widget might be used across many field instances.
Let's look at a widget which renders a text field with an additional class if there are errors:
class MyTextInput(TextInput): def __init__(self, error_class=u'has_errors'): super(MyTextInput, self).__init__() self.error_class = error_class def __call__(self, field, **kwargs): if field.errors: c = kwargs.pop('class', '') or kwargs.pop('class_', '') kwargs['class'] = u'%s %s' % (self.error_class, c) return super(MyTextInput, self).__call__(field, **kwargs)
In the above example, we extended the behavior of the existing :class:`TextInput` widget to append a CSS class as needed. However, widgets need not extend from an existing widget, and indeed don't even have to be a class. For example, here is a widget that renders a :class:`~wtforms.fields.SelectMultipleField` as a collection of checkboxes:
def select_multi_checkbox(field, ul_class='', **kwargs): kwargs.setdefault('type', 'checkbox') field_id = kwargs.pop('id', field.id) html = [u'<ul %s>' % html_params(id=field_id, class_=ul_class)] for value, label, checked in field.iter_choices(): choice_id = u'%s-%s' % (field_id, value) options = dict(kwargs, name=field.name, value=value, id=choice_id) if checked: options['checked'] = 'checked' html.append(u'<li><input %s /> ' % html_options(**options)) html.append(u'<label %s>%s</label></li>') html.append(u'</ul>') return u''.join(html)