[Improvement][Python]: Enhance serialization error messages for better developer experience

[Kalpana-chavhan]

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.

[PDGGK]

Hi, I'd like to work on this issue.

I'll enhance the serialization error messages in PTransformWithSideInputs (ptransform.py) to provide clearer context about pickling failures, including:

• Why serialization is required for distributed execution
• Common causes (lambdas capturing file handles, DB connections, etc.)
• Suggested fixes (module-level functions, checking closures, using DoFn.setup())

.take-issue