Empty Series is considered `TimestampSeries`

[randolf-scholz]

Describe the bug

Instantiating an empty Series object creates a TimestamptSeries, should probably be Series[Any].

To Reproduce

from pandas import Series
baz = Series([])
reveal_type(baz)  # pandas.core.series.TimestampSeries

Please complete the following information:

• pandas 1.5.3
• pandas-stubs 1.5.3.230304

[Dr-Irv]

I'm not sure that this is something that can be fixed. There is not a way for a type checker to differentiate between an empty list that has no type information, and a list/sequence containing specific types.

[randolf-scholz]

@Dr-Irv Right, but then it should fall back to the most general type Series[Any] instead of a highly specialized type TimestampSeries.

I think one could fix this by carefully changing the order of the overloads. The problem is that the first overload of Series.__new__ matches an empty Series and returns TimestampSeries. If I read https://mypy.readthedocs.io/en/stable/more_types.html#type-checking-calls-to-overloads correctly, then if the first overload was e.g. of the form data: Sequence[Scalar] -> Series[Scalar], then mypy would correctly impute Any.

[Dr-Irv]

I think one could fix this by carefully changing the order of the overloads. The problem is that the first overload of Series.__new__ matches an empty Series and returns TimestampSeries. If I read https://mypy.readthedocs.io/en/stable/more_types.html#type-checking-calls-to-overloads correctly, then if the first overload was e.g. of the form data: Sequence[Scalar] -> Series[Scalar], then mypy would correctly impute Any.

The problem with that solution is that we want to have a Sequence[Timestamp] match to create TimestampSeries, (and similar with the Timedelta related types), so we have to use a proper subset of S1 that doesn't include the other types we want to match inside of sequences.

This might be possible if we split S1 into two groups, with one group including the types that we don't want to explicitly match. Not sure - some investigation would be required to get this right.

[randolf-scholz]

Empty Sequence can be handled by Sequence[Never] in pyright, currently it seems unsupported in mypy: python/mypy#15763

So, one might have to wait for newer version of mypy.