Check if the `UNSPECIFIED` module-level sentinel is needed

[Matt711]

Follow-up from PR #23558 review: Unspecified() already always returns a singleton, so it's
unclear whether the module-level UNSPECIFIED instance is still needed, or if Unspecified()
should just be used directly wherever a sentinel is required:

cudf/python/cudf_polars/cudf_polars/utils/config.py

Lines 68 to 93 in d4ecace

	class Unspecified:
	"""
	Sentinel value meaning "no value was explicitly provided".
	The singleton instance :data:`UNSPECIFIED` is used as the default for every
	:class:`StreamingOptions` field, as well as for
	``ParquetOptions.prefetch_file_metadata``. When a field is still
	``UNSPECIFIED`` after construction (i.e. neither an explicit value nor a
	matching environment variable was provided), the consuming component decides
	on the semantics.
	"""
	_instance: Unspecified | None = None
	def __new__(cls) -> Unspecified:
	"""Return the singleton instance."""
	if cls._instance is None:
	cls._instance = super().__new__(cls)
	return cls._instance
	def __repr__(self) -> str:
	"""Return ``"UNSPECIFIED"``."""
	return "UNSPECIFIED"
	UNSPECIFIED = Unspecified()

Related

• PR [PERF]: Improve the performance of plc.io.parquet.read_parquet with prefetched parquet file metadata. #23558
• [PERF]: Improve the performance of plc.io.parquet.read_parquet with prefetched parquet file metadata. #23558 (comment)