apache/beam · error · ValueError

Invalid DisplayDataItem

Error message

Invalid DisplayDataItem %s. Namespace must not be None

What it means

DisplayDataItem.is_valid requires a namespace identifying which component the item belongs to; it raises ValueError when namespace is None. The namespace is usually derived automatically from the owning class.

Solutions

Create items through DisplayData.create_from()/create_from_options() so the namespace is auto-populated.
Set the namespace explicitly when constructing the item manually.
Inspect the item in the error message to find where namespace was lost.

Example fix

// before
item = DisplayDataItem(key='k', value=1)
// after
item = DisplayDataItem(key='k', value=1, namespace='my.module.MyDoFn')

Defensive patterns

Strategy: validation

Validate before calling

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

Type guard

def item_has_namespace(item) -> bool:
    return getattr(item, 'namespace', None) is not None

Try / catch

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

Prevention

Always derive items from a DisplayData object that knows its owner
Do not manually copy DisplayDataItem objects across namespaces
Validate serialized display data in integration tests

When it happens

Trigger: Serializing a DisplayDataItem whose namespace field was never populated (e.g. an item constructed manually or detached from its DisplayData origin) via is_valid()/get_dict().

Common situations: Manually built DisplayDataItem objects missing namespace; copying items between DisplayData objects; deserialization paths that dropped the namespace.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/3fb95e08201157ed. Report an issue: GitHub.

Appendix: source

Thrown at sdks/python/apache_beam/transforms/display.py:341

      return True
    return False

  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:

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