docs: def validate_username(form, field):
          """Wrap username validator for WTForms."""
          try:
              validate_username(field.data)
          except ValueError as e:
              raise ValidationError(e)
  
          try:
              user_profile = UserProfile.get_by_username(field.data)
              if current_userprofile.is_anonymous or \
                      (current_userprofile.user_id != user_profile.user_id and
                       field.data != current_userprofile.username):
                  # NOTE: Form validation error.
                  raise ValidationError(_('Username already exists.'))
          except NoResultFound:
              return