Merge pull request numpy#26317 from ngoldbaum/astype-stringdtype-fix

BUG: use PyArray_SafeCast in array_astype

Author: charris

numpy/_core/src/multiarray/methods.c

@@ -811,8 +811,8 @@ array_astype(PyArrayObject *self,
 
     /*
      * If the memory layout matches and, data types are equivalent,
-     * and it's not a subtype if subok is False, then we
-     * can skip the copy.
+     * it's not a subtype if subok is False, and if the cast says
+     * view are possible, we can skip the copy.
      */
     if (forcecopy != NPY_AS_TYPE_COPY_ALWAYS &&
                     (order == NPY_KEEPORDER ||
@@ -823,11 +823,15 @@ array_astype(PyArrayObject *self,
                         PyArray_IS_C_CONTIGUOUS(self)) ||
                     (order == NPY_FORTRANORDER &&
                         PyArray_IS_F_CONTIGUOUS(self))) &&
-                (subok || PyArray_CheckExact(self)) &&
-                PyArray_EquivTypes(dtype, PyArray_DESCR(self))) {
-        Py_DECREF(dtype);
-        Py_INCREF(self);
-        return (PyObject *)self;
+                (subok || PyArray_CheckExact(self))) {
+        npy_intp view_offset;
+        npy_intp is_safe = PyArray_SafeCast(dtype, PyArray_DESCR(self),
+                                             &view_offset, NPY_NO_CASTING, 1);
+        if (is_safe && (view_offset != NPY_MIN_INTP)) {
+            Py_DECREF(dtype);
+            Py_INCREF(self);
+            return (PyObject *)self;
+        }
     }
 
     if (!PyArray_CanCastArrayTo(self, dtype, casting)) {

numpy/_core/tests/test_stringdtype.py

@@ -493,6 +493,13 @@ def test_create_with_copy_none(string_list):
     assert arr_view is arr
 
 
+def test_astype_copy_false():
+    orig_dt = StringDType()
+    arr = np.array(["hello", "world"], dtype=StringDType())
+    assert not arr.astype(StringDType(coerce=False), copy=False).dtype.coerce
+
+    assert arr.astype(orig_dt, copy=False).dtype is orig_dt
+
 @pytest.mark.parametrize(
     "strings",
     [