Tag to return a truncated chain of user names.

def chain_user_names(users, exclude_user, truncate=35):
    """Tag to return a truncated chain of user names."""
    if not users or not isinstance(exclude_user, get_user_model()):
        return ''
    return truncatechars(
        ', '.join(u'{}'.format(u) for u in users.exclude(pk=exclude_user.pk)),
        truncate)