Should Series[Any] be used internally instead of Series?

[MarcoGorelli]

Currently, Series is used in several places where the inner type of the Series isn't known, e.g.:

pandas-stubs/pandas-stubs/core/series.pyi

Lines 864 to 871 in 25fe8aa

	@overload
	def compare(
	self,
	other: Series,
	align_axis: AxisColumn = ...,
	keep_shape: bool = ...,
	keep_equal: bool = ...,
	) -> DataFrame: ...

There's a couple of issues I'm running into with this

First, the pyright-strict job marks this as partially unknown:

/home/runner/work/pandas-stubs/pandas-stubs/tests/test_series.py:1039:5 - error: Type of "compare" is partially unknown
Type of "compare" is "Overload[(other: Series[Unknown], align_axis: Literal['index', 0], keep_shape: bool = ..., keep_equal: bool = ...) -> Series[Unknown], (other: Series[Unknown], align_axis: Literal['columns', 1] = ..., keep_shape: bool = ..., keep_equal: bool = ...) -> DataFrame]" (reportUnknownMemberType)

Second, when using pyright with --verifytypes to look for uncovered parts of the public API, this is flagged as "unknown type":

            {
                "category": "function",
                "name": "pandas.testing.assert_series_equal",
                "referenceCount": 1,
                "isExported": true,
                "isTypeKnown": false,
                "isTypeAmbiguous": false,
                "diagnostics": [
                    {
                        "file": "/home/marcogorelli/type_coverage_py/.pyright_env_pandas/lib/python3.12/site-packages/pandas/_testing/__init__.pyi",
                        "severity": "error",
                        "message": "Type of parameter \"left\" is partially unknown\n  Parameter type is \"Series[Unknown]\"\n    Type argument 1 for class \"Series\" has unknown type",
                        "range": {
                            "start": {
                                "line": 4,
                                "character": 27
                            },
                            "end": {
                                "line": 4,
                                "character": 46
                            }
                        }
                    },

Would it be OK to use Series[Any] instead of just Series in such cases? Or, as some libraries do, to introduce a type alias Incomplete: TypeAlias = Any to mean "we should be able to narrow down the type but for now we're not doing so" and use that in some cases

The latter use-case (--verifytypes) can, I think, really help to prioritise which stubs to add

[Dr-Irv]

We've had discussion about this in another PR. See #1093 (comment)

Idea is to create an UnknownSeries type that would correspond to when we don't know the type. I think this may solve the problem you raise above.