Fix silent per-char iteration when DoFn returns str/bytes/dict

[chrisqiqiu]

fixes #18712

Background

A user-written DoFn whose process accidentally returns a str/bytes/dict instead of an iterable wrapping one e.g.:

class MyDoFn(beam.DoFn):
    def process(self, element):
        return element + '-processed'   

Silently iterates per-character/byte/key instead of erroring. The string 'abc-processed' becomes 13 single-character output elements ('a', 'b', 'c', ...). The pipeline runs to completion with wrong output, and downstream errors point at the wrong place. This is hard to debug.

This existing check lives in typehints/typecheck.py:_check_type but is gated behind both @with_output_types(...) and --runtime_type_check - so most users never see it.

Fix

Add a cheap isinstance(results, (str, bytes, dict)) guard in _OutputHandler.handle_process_outputs (runners/common.py) that raises a clear TypeError pointing the user at list(...). The error message reuses the wording from the existing _check_type.

Why scoped to user class-based DoFns

The check is gated by a new _check_user_dofn_output flag set at DoFnRunner construction time, computed as not isinstance(fn, core.CallableWrapperDoFn).

This deliberately skips the check for Map/FlatMap callables. In particular, FlatMap() with the default identity function is a documented pattern for flattening iterables - including strings into characters. Blocking str/bytes/dict universally would break that legitimate use case (and the existing FlatMapTest::test_default test).

Tests

pytest \
  apache_beam/transforms/core_test.py::FlatMapTest \
  apache_beam/runners/common_test.py::DoFnProcessTest \
  apache_beam/typehints/typecheck_test.py -v

Three regression tests in runners/common_test.py::DoFnProcessTest covering str, bytes, and dict return values from a class-based DoFn. Verified the previously-cryptic case now raises:

TypeError: Returning a str from a ParDo or FlatMap is discouraged.
Please use list("abc-processed") if you really want this behavior.

Existing typehints/typecheck_test.py (13 tests) and broader transforms/ suite (787 tests) all still pass.

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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 addresses a common usability issue where DoFn methods returning non-iterable types like strings, bytes, or dictionaries cause silent, hard-to-debug pipeline behavior due to implicit iteration. By introducing a targeted validation check, the system now provides immediate feedback to developers, while maintaining compatibility for legitimate use cases in FlatMap operations.

Highlights

• Improved Error Handling: Added a runtime check in _OutputHandler to raise a TypeError when a user-defined DoFn returns a str, bytes, or dict instead of an iterable, preventing silent per-character/byte/key iteration.
• Targeted Validation: The check is scoped specifically to class-based DoFns, intentionally excluding Map/FlatMap callables to preserve existing documented behavior for flattening iterables.
• Regression Testing: Added new test cases in common_test.py to verify that the appropriate TypeError is raised for str, bytes, and dict return types.

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Code Review

This pull request implements a validation check in the Python SDK to prevent DoFn.process from returning str, bytes, or dict types, which previously led to silent per-element iteration bugs. The implementation updates _OutputHandler and adds corresponding regression tests. Feedback focuses on improving the error message formatting using %r for better type representation and tightening test assertions by specifically checking for TypeError instead of a generic Exception.

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