I have created this widget

class DateTimeWidget(forms.TextInput):
  attr = {'class': 'datetimepicker'}
  class Media:
    js = ('js/jquery-ui-timepicker-addon.js',)

Then I use it on my form

class SessionForm(forms.ModelForm):
  class Meta:
    model = Session
  def __init__(self, *args, **kwargs):
    super(SessionForm, self).__init__(*args, **kwargs)
    self.fields['start_time'].widget = DateTimeWidget()
    self.fields['end_time'].widget = DateTimeWidget()

No css class is applied to my fields (I'm expecting datetimepicker applied to both start_time & end_time).
I imagine I have put attr at a wrong location. Where am I supposed to specify it ?