Merge pull request numpy#27057 from ngoldbaum/fix-array-assign-subscript

Author: ngoldbaum

numpy/_core/src/multiarray/mapping.c

@@ -2034,6 +2034,7 @@ array_assign_subscript(PyArrayObject *self, PyObject *ind, PyObject *op)
         goto fail;
     }
 
+    int allocated_array = 0;
     if (tmp_arr == NULL) {
         /* Fill extra op, need to swap first */
         tmp_arr = mit->extra_op;
@@ -2047,6 +2048,7 @@ array_assign_subscript(PyArrayObject *self, PyObject *ind, PyObject *op)
         if (PyArray_CopyObject(tmp_arr, op) < 0) {
              goto fail;
         }
+        allocated_array = 1;
     }
 
     if (PyArray_MapIterCheckIndices(mit) < 0) {
@@ -2090,10 +2092,12 @@ array_assign_subscript(PyArrayObject *self, PyObject *ind, PyObject *op)
         /* May need a generic copy function (only for refs and odd sizes) */
         NPY_ARRAYMETHOD_FLAGS transfer_flags;
         npy_intp itemsize = PyArray_ITEMSIZE(self);
-
-        if (PyArray_GetDTypeTransferFunction(1,
-                itemsize, itemsize,
-                PyArray_DESCR(self), PyArray_DESCR(self),
+        // TODO: the heuristic used here to determine the src_dtype might be subtly wrong
+        // for non-REFCHK user DTypes. See gh-27057 for the prior discussion about this.
+        if (PyArray_GetDTypeTransferFunction(
+                1, itemsize, itemsize,
+                allocated_array ? PyArray_DESCR(mit->extra_op) : PyArray_DESCR(self),
+                PyArray_DESCR(self),
                 0, &cast_info, &transfer_flags) != NPY_SUCCEED) {
             goto fail;
         }

numpy/_core/tests/test_stringdtype.py

@@ -495,13 +495,15 @@ def test_fancy_indexing(string_list):
     sarr = np.array(string_list, dtype="T")
     assert_array_equal(sarr, sarr[np.arange(sarr.shape[0])])
 
-    # see gh-27003
-    for ind in [[0, 1], ...]:
-        a = np.array(['a'*16, 'b'*16], dtype="T")
-        b = np.array(['d'*16, 'e'*16], dtype="T")
-        a[ind] = b
-        assert_array_equal(a, b)
-        assert a[0] == 'd'*16
+    # see gh-27003 and gh-27053
+    for ind in [[True, True], [0, 1], ...]:
+        for lop in [['a'*16, 'b'*16], ['', '']]:
+            a = np.array(lop, dtype="T")
+            rop = ['d'*16, 'e'*16]
+            for b in [rop, np.array(rop, dtype="T")]:
+                a[ind] = b
+                assert_array_equal(a, b)
+                assert a[0] == 'd'*16
 
 
 def test_creation_functions():