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

Author: fjetter

doc/source/whatsnew/v3.0.0.rst

@@ -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`)
 -

pandas/core/arrays/categorical.py

@@ -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
-            result = self._set_dtype(dtype)
+            result = self._set_dtype(dtype, copy=False)
 
         elif isinstance(dtype, ExtensionDtype):
             return super().astype(dtype, copy=copy)
@@ -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
 
@@ -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:

pandas/tests/arrays/categorical/test_astype.py

@@ -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]))