[SPARK-58500][PYTHON] Accept tuple in DataFrame.describe and selectExpr

[Spenserrrr]

What changes were proposed in this pull request?

DataFrame.describe and DataFrame.selectExpr accept their column names / SQL expressions as varargs, and also allow a single sequence to be passed in place of the varargs (e.g. df.describe(["a", "b"])). Today that single-sequence form is unwrapped only when it is a list — the runtime check is isinstance(x[0], list) — so passing a tuple such as df.describe(("a", "b")) is not unwrapped and is instead treated as a single, invalid column argument.

This PR:

• Widens the describe and selectExpr type annotations from Union[str, List[str]] to Union[str, Sequence[str]] (overloads + implementation) across the base (sql/dataframe.py), classic (sql/classic/dataframe.py), and connect (sql/connect/dataframe.py) layers.
• Changes the runtime unwrap check from isinstance(x[0], list) to not isinstance(x[0], str) and isinstance(x[0], Sequence), and unwraps with x = tuple(x[0]), so a single tuple is unpacked the same way a list is.
• Normalizes the overload shape: describe had no overloads in any layer (added the 2-overload shape *x: str / __x: Sequence[str]); selectExpr had a *expr: List[str] overload in base + classic (fixed to a single-arg __expr: Sequence[str]) and none in connect (added). Drops the now-unnecessary # type: ignore[assignment] on the unwrap.

This is a follow-up to the SPARK-58488 cleanup (#57693). Unlike the writer methods (partitionBy/clusterBy/bucketBy/sortBy), which already accepted a list or a tuple, describe/selectExpr accepted a list only, so this adds tuple/sequence support.

Why are the changes needed?

The runtime already intends to accept "a name or a sequence of names", but the check and the annotation only admitted list. Passing a tuple silently did the wrong thing (treated the whole tuple as one column/expression) rather than being unpacked. Accepting any non-str Sequence makes the behavior consistent with the writer methods and with the general varargs-or-sequence convention in the DataFrame API, and makes the type annotation honest about what is accepted.

Does this PR introduce any user-facing change?

Yes. Previously df.describe(("a", "b")) / df.selectExpr(("e1", "e2")) (a single tuple) was not unpacked and did not behave like the list form; now a single tuple of column names / expressions is accepted and unpacked just like a list. This is a backward-compatible addition — the existing str-varargs and single-list forms are unchanged.

How was this patch tested?

• Added tuple-form cases to the existing connect-vs-classic parity tests test_connect_stat.py::test_describe and test_connect_basic.py::test_select_expr (these assert the same call on the classic and connect sessions produce equal results, so they cover both layers). Both pass.
• Full-scope mypy over python/pyspark is clean.
• ruff format --check and ruff check pass on the changed files.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Opus 4.8)

[uros-b]

Thank you @Spenserrrr! cc @Yicong-Huang