BUG: astype('category') on dataframe backed by non-writeable arrays raises ValueError (#53678)

Author: lukemanley

doc/source/whatsnew/v2.1.0.rst

@@ -349,6 +349,7 @@ Bug fixes
 
 Categorical
 ^^^^^^^^^^^
+- Bug in :meth:`Series.astype` with ``dtype="category"`` for nullable arrays with read-only null value masks (:issue:`53658`)
 - Bug in :meth:`Series.map` , where the value of the ``na_action`` parameter was not used if the series held a :class:`Categorical` (:issue:`22527`).
 -
 

pandas/_libs/hashtable_class_helper.pxi.in

@@ -648,7 +648,7 @@ cdef class {{name}}HashTable(HashTable):
             UInt8Vector result_mask
             UInt8VectorData *rmd
             bint use_na_value, use_mask, seen_na = False
-            uint8_t[:] mask_values
+            const uint8_t[:] mask_values
 
         if return_inverse:
             labels = np.empty(n, dtype=np.intp)

pandas/tests/extension/test_categorical.py

@@ -319,3 +319,12 @@ def test_repr_2d(self, data):
 
         res = repr(data.reshape(-1, 1))
         assert res.count("\nCategories") == 1
+
+
+def test_astype_category_readonly_mask_values():
+    # GH 53658
+    df = pd.DataFrame([0, 1, 2], dtype="Int64")
+    df._mgr.arrays[0]._mask.flags["WRITEABLE"] = False
+    result = df.astype("category")
+    expected = pd.DataFrame([0, 1, 2], dtype="Int64").astype("category")
+    tm.assert_frame_equal(result, expected)