pd.Series gets converted to pd.TimestampSeries after isinstance check

[tvdboom]

Describe the bug
A pd.Series variable changes type after an isinstance check.

To Reproduce

1. Provide a minimal runnable pandas example that is not properly checked by the stubs.

df1: pd.Series | pd.DataFrame = pd.Series()
reveal_type(df1)  # <- As expected
if isinstance(df1, pd.Series):
    reveal_type(df1)  # <- ???

3. Indicate which type checker you are using (mypy or pyright).
   mypy
4. Show the error message received from that type checker while checking your example.

note: Revealed type is "Union[pandas.core.series.Series[Any], pandas.core.frame.DataFrame]"
note: Revealed type is "pandas.core.series.TimestampSeries"

Please complete the following information:

• OS: Windows
• OS Version: 10
• python version: 3.11
• version of type checker: 1.8.0
• version of installed pandas-stubs: 2.2.0.240218

[Dr-Irv]

pyright 1.1.351 gets it right:

issue877.py:4:13 - information: Type of "df1" is "Series[Unknown]"
issue877.py:5:4 - error: Unnecessary isinstance call; "Series[Unknown]" is always an instance of "Series[Unknown]" (reportUnnecessaryIsInstance)
issue877.py:6:17 - information: Type of "df1" is "Series[Unknown]"

I think this is a mypy bug. See python/mypy#15322

I'll keep this open in case mypy ever fixes that issue.