pandas Series.reset_index() returning wrong type when index of Series is a MultiIndex

[Dr-Irv]

• Language Server version: 2020.9.5
• OS and version: Windows 10
• Python version (& distribution if applicable, e.g. Anaconda): Anaconda Python 3.7.5

Expected behaviour

No error

Actual behaviour

mi3: pd.Index = pd.MultiIndex.from_product(
    [[1, 2], [3, 4], [5, 6]], names=["a", "b", "c"]
)
s = pd.Series([i for i in range(len(mi3))], index=mi3)
dr: pd.DataFrame = s.reset_index(-1)
print("dr")
print(dr)
print(type(dr))

reports for line 5

Expression of type "Series[Unknown]" cannot be assigned to declared type "DataFrame"
  "Series[Unknown]" is incompatible with "DataFrame"

The issue here is that reset_index() can return a Series or DataFrame dependent on whether the parameter drop is True or False. (It is False by default). In the example above, it is returning a DataFrame

[Dr-Irv]

Here's a fix:

diff series.pyi.ORIG series.pyi
189c190
<         self, level: Optional[Level] = ..., drop: _bool = ..., name: Optional[object] = ..., inplace: _bool = ...,
---
>         self, level: Optional[Level] = ..., drop: _bool = Literal[True], name: Optional[object] = ..., inplace: _bool = ...,
191a193,196
>     def reset_index(
>         self, level: Optional[Level] = ..., drop: _bool = ..., name: Optional[object] = ..., inplace: _bool = ...,
>     ) -> DataFrame: ...
>     @overload

[gramster]

Thanks for all these reports and suggested fixes; that's awesome!

[jakebailey]

This issue has been fixed in version 2020.9.6, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/master/CHANGELOG.md#202096-23-september-2020