[Improvement][Python]: Enhance serialization error messages for better developer experience #37209
Description
Description
Apache Beam Python SDK requires user-defined functions to be serializable for distributed execution. Currently, when users pass non-serializable lambdas or closures (e.g., capturing a file handle or a database connection), the resulting error is a low-level PicklingError or AttributeError that does not explain the context.
Proposed Solution
Interpose a more descriptive RuntimeError during the serialization check in PTransformWithSideInputs (and potentially pickler.py).
The improved message should
- Identify that a serialization failure occurred during pipeline construction.
- Explain the requirement for 'picklable' functions in distributed processing.
- Suggest common fixes (e.g., using module-level functions, checking closure captures, or using DoFn.setup()).
Impact:
Developer Experience: Significantly reduces "head-scratching" time for new users.
Stability: No change to execution logic; pure diagnostic improvement.
Compatibility: No impact on existing pipelines.