docs: def validate_equal(value):
      """
      Validate the field value is equal to the given value.
      Should work with anything that supports '==' operator.
  
      :param value: Value to compare.
      :raises: ``ValidationError('equal')``
      """
      def equal_validator(field, data):
          if field.value is None:
              return
          if not (field.value == value):
              raise ValidationError('equal', other=value)
      return equal_validator