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BUG: Avoid copying categorical codes if copy=False #62000

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Merged
merged 7 commits into from
Aug 1, 2025
+14 −3
Merged

BUG: Avoid copying categorical codes if copy=False #62000

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f7c4694
Avoid copying categoricals
fjetter Jul 30, 2025
9192881
Add a test
fjetter Jul 31, 2025
b43e5ba
add whatsnew entry
fjetter Jul 31, 2025
b3bc7a4
Add issue as comment
fjetter Jul 31, 2025
aff3cd4
remove unnecessary assert
fjetter Jul 31, 2025
b9b1d25
use tm.shares_memory
fjetter Aug 1, 2025
673b247
Merge branch 'main' into avoid_copy_categoricls
fjetter Aug 1, 2025
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1 change: 1 addition & 0 deletions doc/source/whatsnew/v3.0.0.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -687,6 +687,7 @@ Bug fixes
Categorical
^^^^^^^^^^^
- Bug in :func:`Series.apply` where ``nan`` was ignored for :class:`CategoricalDtype` (:issue:`59938`)
- Bug in :meth:`Categorical.astype` where ``copy=False`` would still trigger a copy of the codes (:issue:`62000`)
- Bug in :meth:`DataFrame.pivot` and :meth:`DataFrame.set_index` raising an ``ArrowNotImplementedError`` for columns with pyarrow dictionary dtype (:issue:`53051`)
- Bug in :meth:`Series.convert_dtypes` with ``dtype_backend="pyarrow"`` where empty :class:`CategoricalDtype` :class:`Series` raised an error or got converted to ``null[pyarrow]`` (:issue:`59934`)
-
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8 changes: 5 additions & 3 deletions pandas/core/arrays/categorical.py
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Expand Up @@ -575,7 +575,7 @@ def astype(self, dtype: AstypeArg, copy: bool = True) -> ArrayLike:
# GH 10696/18593/18630
dtype = self.dtype.update_dtype(dtype)
self = self.copy() if copy else self
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Do we need this line if we pass self._set_dtype(dtype, copy=copy)?

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I wasn't entirely sure. The self.copy is also creating a new Categorical object which is desired, I assume.

result = self._set_dtype(dtype)
result = self._set_dtype(dtype, copy=False)

elif isinstance(dtype, ExtensionDtype):
return super().astype(dtype, copy=copy)
Expand Down Expand Up @@ -945,7 +945,7 @@ def _set_categories(self, categories, fastpath: bool = False) -> None:

super().__init__(self._ndarray, new_dtype)

def _set_dtype(self, dtype: CategoricalDtype) -> Self:
def _set_dtype(self, dtype: CategoricalDtype, copy: bool = True) -> Self:
"""
Internal method for directly updating the CategoricalDtype

Expand All @@ -958,7 +958,9 @@ def _set_dtype(self, dtype: CategoricalDtype) -> Self:
We don't do any validation here. It's assumed that the dtype is
a (valid) instance of `CategoricalDtype`.
"""
codes = recode_for_categories(self.codes, self.categories, dtype.categories)
codes = recode_for_categories(
self.codes, self.categories, dtype.categories, copy
)
return type(self)._simple_new(codes, dtype=dtype)

def set_ordered(self, value: bool) -> Self:
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8 changes: 8 additions & 0 deletions pandas/tests/arrays/categorical/test_astype.py
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Expand Up @@ -130,6 +130,14 @@ def test_astype_category(self, dtype_ordered, ordered):
expected = cat
tm.assert_categorical_equal(result, expected)

def test_astype_category_copy_false_nocopy_codes(self):
# GH#62000
cat = Categorical([3, 2, 4, 1])
new = cat.astype("category", copy=False)
assert tm.shares_memory(new.codes, cat.codes)
new = cat.astype("category", copy=True)
assert not tm.shares_memory(new.codes, cat.codes)

def test_astype_object_datetime_categories(self):
# GH#40754
cat = Categorical(to_datetime(["2021-03-27", NaT]))
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