[Python] Fix AttributeError in ExternalTransform.expand by using get_type_hints()

[Eliaaazzz]

Description

Fixes #37289 ： Replaces direct access to the internal _type_hints attribute with the public get_type_hints() accessor in ExternalTransform.expand().

Why this is needed

Directly accessing self._type_hints causes an AttributeError if the type hints have not been explicitly initialized before the expansion phase. This issue was observed specifically when using StorageWriteToBigQuery with with_auto_sharding=True, which triggers an ExternalTransform expansion where _type_hints is missing.

Using get_type_hints() is the correct, defensive approach as it ensures valid type hints (or an empty instance) are returned, preventing the crash.

Testing

• Reproduced the crash locally using a script involving StorageWriteToBigQuery with auto-sharding enabled.
• Verified that the fix resolves the AttributeError and allows the pipeline to proceed past the expansion phase.

Checklist

• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

[gemini-code-assist]

Summary of Changes

Hello @Eliaaazzz, 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 resolves a critical AttributeError that could arise during the expansion phase of ExternalTransform when the internal _type_hints attribute was accessed before its initialization. By adopting the more robust get_type_hints() method, the system now defensively retrieves type hints, ensuring pipeline stability and preventing crashes in scenarios where type hints might be absent or not yet set up, thereby improving the reliability of data processing operations.

Highlights

• AttributeError Fix: Replaced direct access to the internal self._type_hints attribute with the public self.get_type_hints() method within ExternalTransform.expand(). This change prevents an AttributeError that occurred when _type_hints was not explicitly initialized, particularly impacting StorageWriteToBigQuery with auto-sharding.

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[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 55.17%. Comparing base (4dcc27d) to head (8fe6cc6).

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/transforms/external.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #37299      +/-   ##
============================================
+ Coverage     55.16%   55.17%   +0.01%     
  Complexity     1676     1676              
============================================
  Files          1068     1068              
  Lines        167257   167342      +85     
  Branches       1208     1208              
============================================
+ Hits          92261    92338      +77     
- Misses        72816    72824       +8     
  Partials       2180     2180              

Flag	Coverage Δ
python	80.79% <80.00%> (+0.01%)	⬆️

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[github-actions]

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R: @jrmccluskey for label python.

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