old_contents: """
  Based on http://www.djangosnippets.org/snippets/595/
  by sopelkin
  """
  
  from django import forms
  from django.forms import widgets
  from django.contrib.auth.models import User
  from django.utils.translation import ugettext_lazy as _
  
  
  class CommaSeparatedUserInput(widgets.Input):
      input_type = 'text'
      
      def render(self, name, value, attrs=None):
          if value is None:
              value = ''
          elif isinstance(value, (list, tuple)):
              value = (', '.join([user.username for user in value]))
          return super(CommaSeparatedUserInput, self).render(name, value, attrs)
          
  
  
  class CommaSeparatedUserField(forms.Field):
      widget = CommaSeparatedUserInput
      
      def __init__(self, *args, **kwargs):
          recipient_filter = kwargs.pop('recipient_filter', None)
          self._recipient_filter = recipient_filter
          super(CommaSeparatedUserField, self).__init__(*args, **kwargs)
          
      def clean(self, value):
          super(CommaSeparatedUserField, self).clean(value)
          if not value:
              return ''
          if isinstance(value, (list, tuple)):
              return value
          
          names = set(value.split(','))
          names_set = set([name.strip() for name in names])
          users = list(User.objects.filter(username__in=names_set))
          unknown_names = names_set ^ set([user.username for user in users])
          
          recipient_filter = self._recipient_filter
          invalid_users = []
          if recipient_filter is not None:
              for r in users:
                  if recipient_filter(r) is False:
                      users.remove(r)
                      invalid_users.append(r.username)
          
          if unknown_names or invalid_users:
              raise forms.ValidationError(_(u"The following usernames are incorrect: %(users)s") % {'users': ', '.join(list(unknown_names)+invalid_users)})
          
          return users
  
  
  
new_contents: """
  Based on http://www.djangosnippets.org/snippets/595/
  by sopelkin
  """
  
  from django import forms
  from django.forms import widgets
  from django.contrib.auth.models import User
  from django.utils.translation import ugettext_lazy as _
  
  
  class CommaSeparatedUserInput(widgets.Input):
      input_type = 'text'
      
      def render(self, name, value, attrs=None):
          if value is None:
              value = ''
          elif isinstance(value, (list, tuple)):
              value = (', '.join([user.username for user in value]))
          return super(CommaSeparatedUserInput, self).render(name, value, attrs)
          
  
  
  class CommaSeparatedUserField(forms.Field):
      widget = CommaSeparatedUserInput
      
      def __init__(self, *args, **kwargs):
          recipient_filter = kwargs.pop('recipient_filter', None)
          self._recipient_filter = recipient_filter
          super(CommaSeparatedUserField, self).__init__(*args, **kwargs)
          
      def clean(self, value):
          super(CommaSeparatedUserField, self).clean(value)
          if not value:
              return ''
          if isinstance(value, (list, tuple)):
              return value
          
          names = set(value.split(','))
          names_set = set([name.strip() for name in names if name.strip()])
          users = list(User.objects.filter(username__in=names_set))
          unknown_names = names_set ^ set([user.username for user in users])
          
          recipient_filter = self._recipient_filter
          invalid_users = []
          if recipient_filter is not None:
              for r in users:
                  if recipient_filter(r) is False:
                      users.remove(r)
                      invalid_users.append(r.username)
          
          if unknown_names or invalid_users:
              raise forms.ValidationError(_(u"The following usernames are incorrect: %(users)s") % {'users': ', '.join(list(unknown_names)+invalid_users)})
          
          return users
  
  
  
subject: handle only non-empty usernames in CommaSeparatedUserField.clean. Thanks for the fix to novotny.radim. Fixes issue #51.