apache/beam · error · BeamAssertException

Failed assert: Received element

Error message

Failed assert: Received element {} is not of type TestWindowedValue. Did you forget to set reify_windows=True on the assertion?

What it means

equal_to_per_window requires the asserted PCollection to contain TestWindowedValue objects (i.e., windows reified into the elements). Its __call__ type-checks each received item; plain values mean windows were not reified, so per-window matching is impossible, and BeamAssertException is raised with a hint to set reify_windows=True.

Solutions

1. Pass reify_windows=True to assert_that: assert_that(pcoll, equal_to_per_window(expected), reify_windows=True).
2. Ensure nothing between the pipeline and the assertion discards windowing information.
3. If per-window assertions aren't needed, use plain equal_to instead.

Example fix

// before
assert_that(pcoll, equal_to_per_window({win: ['a']}))
// after
assert_that(pcoll, equal_to_per_window({win: ['a']}), reify_windows=True)

# ensure the assertion is wired with reified windows
assert_that(pcoll, equal_to_per_window(expected), reify_windows=True)

from apache_beam.testing.util import TestWindowedValue
def is_test_windowed_value(x) -> bool:
    return isinstance(x, TestWindowedValue)

from apache_beam.testing.util import BeamAssertException
try:
    assert_that(pcoll, equal_to_per_window(expected), reify_windows=True)
except BeamAssertException as e:
    if 'not of type TestWindowedValue' in str(e):
        logging.error('Add reify_windows=True: %s', e)
    else:
        raise

When it happens

Trigger: Calling assert_that(pcoll, equal_to_per_window({...})) WITHOUT reify_windows=True; the assertion receives plain elements (or non-TestWindowedValue objects) and fails on the first one.

Common situations: Copy-pasting an ordinary assert_that/pcol equal_to assertion and swapping in equal_to_per_window without adding the reify_windows flag; using a runner/DoFn that strips window information before the assertion.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Failed assert: element
• Failed assert: unmatched elements
• Failed assert: window
• Failed assert: [] == %r
• Failed assert: %r == %r

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/8e47da2fd04eca2a. Report an issue: GitHub.

Appendix: source

Thrown at sdks/python/apache_beam/testing/util.py:133

        raise BeamAssertException(
            'Failed assert: window {} not found in any expected ' \
            'windows {}'.format(window_key, list(_expected.keys())))\

      # Remove any matched elements from the window. This is used later on to
      # assert that all elements in the window were matched with actual
      # elements.
      try:
        _expected[window_key].remove(actual)
      except ValueError:
        raise BeamAssertException(
            'Failed assert: element {} not found in window ' \
            '{}:{}'.format(actual, window_key, _expected[window_key]))\

    # Run the matcher for each window and value pair. Fails if the
    # windowed_value is not a TestWindowedValue.
    for windowed_value in value:
      if not isinstance(windowed_value, TestWindowedValue):
        raise BeamAssertException(
            'Failed assert: Received element {} is not of type ' \
            'TestWindowedValue. Did you forget to set reify_windows=True ' \
            'on the assertion?'.format(windowed_value))
      match(windowed_value)

    # Finally, some elements may not have been matched. Assert that we removed
    # all the elements that we received from the expected list. If the list is
    # non-empty, then there are unmatched elements.
    for win in _expected:
      if _expected[win]:
        raise BeamAssertException(
            'Failed assert: unmatched elements {} in window {}'.format(
                _expected[win], win))


def equal_to_per_window(expected_window_to_elements):
  """Matcher used by assert_that to check to assert expected windows.

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