Handle some cases during infer schema from dataclass by Abacn · Pull Request #37855 · apache/beam

Abacn · 2026-03-13T21:29:00Z

Follow up #37728 added support for inferring schema from dataclasses

For backward compatibility, only infer schema for frozen dataclasses when it's registered with row coder
Previously frozen dataclass is accepted by FastPrimitiveCoder (while non-frozen will raise an Error). Coder change isn't upgrade-compatible
Make sure Beam schema ID does not inherit
If dataclass is inherited, the subclass assumes Beam Schema ID from base class, which isn't correct.
Fix IndexOutofBoundError trying to infer type from custom Iterable without type hint
the change exposed another issue (previously could also happen for named tuple) for user types defined __iter__ but does not have a typehint for element type.
Found a pre-existing bug [Bug]: [Python] Union of named fields loss fields after GBK #37862. This change fixes named tuple and effectively fails dataclass inside union typehint (instead of silent data corruption if triggered the bug)

internal tracker b/492300593

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gemini-code-assist · 2026-03-13T21:29:15Z

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the robustness and correctness of schema inference within Apache Beam's type hinting system. It specifically refines how schemas are generated for dataclasses by preventing unintended inheritance of schema IDs from base classes, thereby ensuring type-specific schema generation. Additionally, it introduces a defensive check during type inference for generic collections like Iterable and Sequence to prevent errors when type arguments are missing, improving the stability of the schema generation process.

Highlights

Beam Schema ID Inheritance: Implemented logic to prevent _BEAM_SCHEMA_ID from inheriting from base classes when inferring schemas for derived dataclasses, ensuring each type has a unique and correct schema identifier.
IndexOutOfBoundsError Fix: Addressed a potential IndexOutOfBoundsError when inferring types from custom Iterable or Sequence types that might not have explicit type hints, by safely checking for the presence of type arguments.

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Changelog

sdks/python/apache_beam/typehints/row_type.py
- Modified RowTypeConstraint initialization to unset _schema_id if it was inherited from a base class, ensuring it's only set if explicitly defined on the current type.
sdks/python/apache_beam/typehints/row_type_test.py
- Added a new test case test_derived_dataclass_schema_id to verify that _BEAM_SCHEMA_ID is not inherited by derived dataclasses.
- Imported the schemas module for use in the new test.
sdks/python/apache_beam/typehints/schemas.py
- Updated typing_to_runner_api to check the length of _get_args(type_) before accessing its elements for Sequence and Iterable types, preventing IndexOutOfBoundsError.
- Changed the condition for checking _BEAM_SCHEMA_ID in schema_from_element_type from hasattr to _BEAM_SCHEMA_ID in element_type.__dict__ to explicitly prevent inheritance from base classes.

Activity

Abacn created this pull request to address issues with schema inference from dataclasses.
The pull request description outlines two main objectives: preventing Beam schema ID inheritance and fixing an IndexOutOfBoundsError.

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github-actions · 2026-03-13T22:11:46Z

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github-actions · 2026-03-14T03:02:16Z

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* For backward compatibility, only infer schema for frozen dataclasses when it's registered with row coder * Make sure Beam schema ID does not inherit * Fix IndexOutofBoundError trying to infer type from custom Iterable without type hint * Fix apache#37862: fixed named tuple and effectively fails dataclass inside union typehint

Abacn · 2026-03-16T13:58:20Z

sdks/python/apache_beam/typehints/native_type_compatibility.py


-def match_is_dataclass(user_type):
-  return dataclasses.is_dataclass(user_type) and isinstance(user_type, type)
+def match_dataclass_for_row(user_type):


This part fixes #1-reassure backward compatibility of default coder for frozen dataclass

Abacn · 2026-03-16T13:59:28Z

sdks/python/apache_beam/typehints/row_type.py

    # Currently registration happens when converting to schema protos, in
    # apache_beam.typehints.schemas
    self._schema_id = getattr(self._user_type, _BEAM_SCHEMA_ID, None)
+    if self._schema_id and _BEAM_SCHEMA_ID not in self._user_type.__dict__:


this part (and another evaluation of _BEAM_SCHEMA_ID in schema.py) fixes #2 handle inherited data classes

Abacn · 2026-03-16T14:00:13Z

sdks/python/apache_beam/typehints/schemas.py

-      return schema_pb2.FieldType(
-          array_type=schema_pb2.ArrayType(element_type=element_type))
+      arg_types = _get_args(type_)
+      if len(arg_types) > 0:


this (and the following) fixes #3 IndexOutofBoundError

Abacn · 2026-03-16T14:01:37Z

sdks/python/apache_beam/typehints/schemas.py

  element_types must be a set of schema-aware types whose fields have the
  same naming and ordering.
  """
+  named_fields_and_types = []


this part fixes #4 data corruption bug #37862

Abacn · 2026-03-16T14:06:58Z

sdks/python/apache_beam/typehints/row_type_test.py

+
+  # Union of dataclasses as type hint currently result in FastPrimitiveCoder
+  # fails at GBK
+  @unittest.skip("https://github.com/apache/beam/issues/22085")


The more I dig into, more gaps related to typehint<->schema found. This skipped test (dataclass counterpart of namedtuple test above) demonstrates current failure due to CoderRegistry.get_coder does not handle UnionTypeConstraint:

beam/sdks/python/apache_beam/coders/typecoders.py

Line 154 in 487696c

if isinstance(typehint, typehints.IterableTypeConstraint):

and it always falls back to FastPrimitiveCoder, which cannot encode non-frozen dataclass. Even if it does, it's not portable (backed by pickle)

Decide to stop here for this Beam release, as this PR is sufficient to basic dataclass support, in a backward compatibility way, and fixed two pre-existing bug currently also happening for named tuples

Abacn · 2026-03-16T14:08:13Z

R: @claudevdm @tarun-google

github-actions · 2026-03-16T14:09:37Z

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control. If you'd like to restart, comment assign set of reviewers

tarun-google · 2026-03-16T14:57:40Z

LGTM! I would defer approval review to some one with knowledge of schema ID for more cases.

github-actions bot added the python label Mar 13, 2026

Abacn force-pushed the fix-dataclass-cases branch from 701c97d to 0129b49 Compare March 13, 2026 21:31

Abacn force-pushed the fix-dataclass-cases branch from 0129b49 to dc89818 Compare March 13, 2026 22:30

github-actions bot added the Next Action: Reviewers label Mar 14, 2026

Abacn force-pushed the fix-dataclass-cases branch 2 times, most recently from bb4cac3 to f54be68 Compare March 15, 2026 15:50

Abacn mentioned this pull request Mar 15, 2026

[Bug]: [Python] Union of named fields loss fields after GBK #37862

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Abacn force-pushed the fix-dataclass-cases branch from 63fce82 to 18199f1 Compare March 15, 2026 21:05

Abacn force-pushed the fix-dataclass-cases branch from 18199f1 to ed6e34b Compare March 15, 2026 21:08

Abacn commented Mar 16, 2026

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Handle some cases during infer schema from dataclass#37855

Handle some cases during infer schema from dataclass#37855
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