MINOR: [Python][Parquet] Clarify code around restoring pandas metadata in ParquetDataset.read()

[jorisvandenbossche]

Rationale for this change

This might not strictly speaking be a valid "minor" issue since it changes code, but it should keep the logic the same and (for me) clarify the intent of it.

Are these changes tested?

By existing tests

Are there any user-facing changes?

No

[jorisvandenbossche]

If metadata would come from metadata = self.schema.metadata, it means it already had a "pandas" key, and there is no need to add it back here (to_table() does not (or no longer) loose the metadata)

[Copilot]

🟢 Approval recommended

The change is a small, localized refactor that preserves intended behavior while making the metadata source/restore path more explicit.

Pull request overview

This PR refactors ParquetDataset.read() to make the pandas-metadata restoration logic clearer, specifically distinguishing the case where pandas metadata is sourced from _common_metadata / _metadata rather than from the dataset schema itself.

Changes:

• Introduces common_metadata to explicitly track when pandas metadata is obtained from common metadata files for backwards compatibility.
• Narrows the schema-metadata restoration step to only run when pandas metadata was sourced from common metadata (so it won’t already be present on the resulting table’s schema metadata).

File summaries

File	Description
python/pyarrow/parquet/core.py	Clarifies pandas metadata handling by tracking and conditionally re-attaching pandas schema metadata only when it was obtained from common metadata files.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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