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Update types for Series.unstack, DataFrame.stack, and DataFrame.unstack #1259

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Merged
merged 3 commits into from
Jun 20, 2025
+18 −5
Merged

Update types for Series.unstack, DataFrame.stack, and DataFrame.unstack #1259

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8794b35
update types, add tests
Jun 20, 2025
be32ee1
remove unnecessary test
Jun 20, 2025
486f888
added check
Jun 20, 2025
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6 changes: 3 additions & 3 deletions pandas-stubs/core/frame.pyi
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Original file line number Diff line number Diff line change
Expand Up @@ -1317,18 +1317,18 @@ class DataFrame(NDFrame, OpsMixin, _GetItemHack):
) -> Self: ...
@overload
def stack(
self, level: Level | list[Level] = ..., dropna: _bool = ..., sort: _bool = ...
self, level: IndexLabel = ..., dropna: _bool = ..., sort: _bool = ...
) -> Self | Series: ...
@overload
def stack(
self, level: Level | list[Level] = ..., future_stack: _bool = ...
self, level: IndexLabel = ..., future_stack: _bool = ...
) -> Self | Series: ...
def explode(
self, column: Sequence[Hashable], ignore_index: _bool = ...
) -> Self: ...
def unstack(
self,
level: Level = ...,
level: IndexLabel = ...,
fill_value: Scalar | None = ...,
sort: _bool = ...,
) -> Self | Series: ...
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2 changes: 1 addition & 1 deletion pandas-stubs/core/series.pyi
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Expand Up @@ -934,7 +934,7 @@ class Series(IndexOpsMixin[S1], NDFrame):
def explode(self) -> Series[S1]: ...
def unstack(
self,
level: Level = ...,
level: IndexLabel = ...,
fill_value: int | _str | dict | None = ...,
) -> DataFrame: ...
@overload
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1 change: 1 addition & 0 deletions tests/test_frame.py
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Expand Up @@ -4461,6 +4461,7 @@ def test_unstack() -> None:
).set_index([0, 1])

check(assert_type(df.unstack(0), pd.DataFrame | pd.Series), pd.DataFrame)
check(assert_type(df.unstack([0]), pd.DataFrame | pd.Series), pd.DataFrame)
check(
assert_type(
df.unstack(1, fill_value=pd.Timestamp(2023, 4, 5)), pd.DataFrame | pd.Series
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14 changes: 13 additions & 1 deletion tests/test_series.py
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Expand Up @@ -3902,11 +3902,23 @@ def test_series_items() -> None:


def test_cumsum_timedelta() -> None:

s = pd.Series(pd.to_timedelta([1, 2, 3], "h"))
check(assert_type(s.cumsum(), "TimedeltaSeries"), pd.Series, pd.Timedelta)
check(
assert_type(pd.Timestamp(0) + s.cumsum(), "TimestampSeries"),
pd.Series,
pd.Timestamp,
)


def test_series_unstack() -> None:
df = pd.DataFrame([[1, 3, 5], [2, 4, 6]])
s = df.transpose().stack([*range(df.index.nlevels)])
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Can you do check(assert_type(s, pd.Series), pd.Series, np.int64) here? (I think that's the right type)

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That fails with:

tests/test_series.py:3917: error: Expression is of type "DataFrame | Series[Any]", not "Series[Any]"  [assert-type]
Found 1 error in 1 file (checked 224 source files)

What I can do instead is:

def test_series_unstack() -> None:
    df = pd.DataFrame([[1, 3, 5], [2, 4, 6]])
    s = cast(pd.Series, df.transpose().stack([*range(df.index.nlevels)]))
    check(assert_type(s, pd.Series), pd.Series, np.int64)
    check(
        assert_type(
            s.unstack([*range(s.index.nlevels // 2)]),
            pd.DataFrame,
        ),
        pd.DataFrame,
    )

where I explicitly cast the stacked object to a Series. Or would you rather have something else there?

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Replace s = cast(pd.Series, df.transpose().stack([*range(df.index.nlevels)])) with

s = df.transpose().stack([*range(df.index.nlevels)]))
check(assert_type(s, Union[pd.Series, pd.DataFrame), pd.Series)

This then says from a typing perspective, you could get the Union, but at runtime, you will get a Series

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Makes sense -- updated the test.

check(assert_type(s, Union[pd.Series, pd.DataFrame]), pd.Series)
check(
assert_type(
s.unstack([*range(s.index.nlevels // 2)]),
Union[pd.Series, pd.DataFrame],
),
pd.DataFrame,
)