apache/beam · error · ValueError

Invalid DisplayDataItem

Error message

Invalid DisplayDataItem %s. Value must not be None

What it means

DisplayDataItem.is_valid found self.value is None: display data items must carry an actual value (plus key, namespace, and type) to render in the Dataflow UI; this guard fires when an item was built without calling with_value or when the value expression evaluated to None.

Solutions

  1. Provide a concrete value: use a default (e.g. self.n or 0) instead of a possibly-None attribute.
  2. Omit the item entirely when the value is None (build the dict conditionally).
  3. Convert None to a sentinel string if the presence of the key matters.

Example fix

// before
def display_data(self):
    return {'threshold': self.threshold}  # may be None
// after
def display_data(self):
    dd = {}
    if self.threshold is not None:
        dd['threshold'] = self.threshold
    return dd
Defensive patterns

Strategy: validation

Validate before calling

for item in display_data.items:
    if getattr(item, 'value', None) is None:
        raise ValueError('display data item missing value')

Type guard

def item_has_value(item) -> bool:
    return getattr(item, 'value', None) is not None

Try / catch

try:
    d = display_data.get_dict()
except ValueError as e:
    if 'Value must not be None' in str(e):
        drop_none_valued_items(display_data)
        d = display_data.get_dict()
    else:
        raise

Prevention

When it happens

Trigger: display_data() returns an item whose value resolves to None (e.g. an unset attribute used directly as the value) and the DisplayData is later validated via get_dict()/is_valid().

Common situations: Returning an uninitialized instance attribute from display_data(); optional config values that end up None; constructing DisplayDataItem with value=None explicitly.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/2d6c6ff44b4969d1. Report an issue: GitHub.

Appendix: source

Thrown at sdks/python/apache_beam/transforms/display.py:344

  def is_valid(self):
    # type: () -> None

    """ Checks that all the necessary fields of the :class:`DisplayDataItem`
    are filled in. It checks that neither key, namespace, value or type are
    :data:`None`.

    Raises:
      ValueError: If the item does not have a key, namespace,
        value or type.
    """
    if self.key is None:
      raise ValueError(
          'Invalid DisplayDataItem %s. Key must not be None.' % self)
    if self.namespace is None:
      raise ValueError(
          'Invalid DisplayDataItem %s. Namespace must not be None' % self)
    if self.value is None:
      raise ValueError(
          'Invalid DisplayDataItem %s. Value must not be None' % self)
    if self.type is None:
      raise ValueError(
          'Invalid DisplayDataItem. Value {} is of an unsupported type.'.format(
              self.value))

  def _get_dict(self):
    res = {
        'key': self.key,
        'namespace': self.namespace,
        'type': self.type if self.type != 'CLASS' else 'STRING'
    }
    # TODO: Python Class types should not be special-cased once
    # the Fn API is in.
    if self.url is not None:
      res['url'] = self.url
    if self.shortValue is not None:
      res['shortValue'] = self.shortValue

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