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String dtype: keep select_dtypes(include=object) selecting string columns #62323

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Merged
merged 3 commits into from
Sep 21, 2025
Merged

String dtype: keep select_dtypes(include=object) selecting string columns #62323

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866075d
String dtype: keep select_dtypes(include=object) selecting string col…
jorisvandenbossche Sep 12, 2025
535d121
Merge remote-tracking branch 'upstream/main' into string-dtype-select…
jorisvandenbossche Sep 21, 2025
46c0731
add whatsnew
jorisvandenbossche Sep 21, 2025
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4 changes: 3 additions & 1 deletion pandas/core/dtypes/cast.py
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Original file line number Diff line number Diff line change
Expand Up @@ -865,7 +865,9 @@ def invalidate_string_dtypes(dtype_set: set[DtypeObj]) -> None:
np.dtype("<U").type, # type: ignore[arg-type]
}
if non_string_dtypes != dtype_set:
raise TypeError("string dtypes are not allowed, use 'object' instead")
raise TypeError(
"numpy string dtypes are not allowed, use 'str' or 'object' instead"
)


def coerce_indexer_dtype(indexer, categories) -> np.ndarray:
Expand Down
12 changes: 8 additions & 4 deletions pandas/core/frame.py
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Expand Up @@ -5146,10 +5146,14 @@ def check_int_infer_dtype(dtypes):
def dtype_predicate(dtype: DtypeObj, dtypes_set) -> bool:
# GH 46870: BooleanDtype._is_numeric == True but should be excluded
dtype = dtype if not isinstance(dtype, ArrowDtype) else dtype.numpy_dtype
return issubclass(dtype.type, tuple(dtypes_set)) or (
np.number in dtypes_set
and getattr(dtype, "_is_numeric", False)
and not is_bool_dtype(dtype)
return (
issubclass(dtype.type, tuple(dtypes_set))
or (
np.number in dtypes_set
and getattr(dtype, "_is_numeric", False)
and not is_bool_dtype(dtype)
)
or (dtype.type is str and np.object_ in dtypes_set)
)

def predicate(arr: ArrayLike) -> bool:
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18 changes: 10 additions & 8 deletions pandas/tests/frame/methods/test_select_dtypes.py
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Expand Up @@ -102,6 +102,10 @@ def test_select_dtypes_include_using_list_like(self, using_infer_string):
ri = df.select_dtypes(include=[str])
tm.assert_frame_equal(ri, ei)

ri = df.select_dtypes(include=["object"])
ei = df[["a"]]
tm.assert_frame_equal(ri, ei)

def test_select_dtypes_exclude_using_list_like(self):
df = DataFrame(
{
Expand Down Expand Up @@ -309,17 +313,15 @@ def test_select_dtypes_not_an_attr_but_still_valid_dtype(self, using_infer_strin
df["g"] = df.f.diff()
assert not hasattr(np, "u8")
r = df.select_dtypes(include=["i8", "O"], exclude=["timedelta"])
if using_infer_string:
e = df[["b"]]
else:
e = df[["a", "b"]]
# if using_infer_string:
# TODO warn
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in the issue you said And in any case, we should probably still add a warning to pandas 2.3 about this when the string mode is enabled (for if we do a 2.3.2 release)

so this TODO should this be part of this PR?

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This PR just restores the old behaviour, and in that case we don't need to warn, I think. We should add a warning that "object" will stop selecting string columns at some point, but in any case I want to only do that in a later PR because that is a lot more complicated.

It was for the case that we decided to keep the current-main behaviour of object not selecting string columns that we should have definitely added a warning to warn users that they are not getting the result they are expecting.

e = df[["a", "b"]]
tm.assert_frame_equal(r, e)

r = df.select_dtypes(include=["i8", "O", "timedelta64[ns]"])
if using_infer_string:
e = df[["b", "g"]]
else:
e = df[["a", "b", "g"]]
# if using_infer_string:
# TODO warn
e = df[["a", "b", "g"]]
tm.assert_frame_equal(r, e)

def test_select_dtypes_empty(self):
Expand Down
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