Merge pull request numpy#27715 from ngoldbaum/fix-generic-fancy-index-cast

BUG: fix incorrect output descriptor in fancy indexing

Author: ngoldbaum

numpy/_core/src/multiarray/mapping.c

@@ -1668,7 +1668,7 @@ array_subscript(PyArrayObject *self, PyObject *op)
 
         if (PyArray_GetDTypeTransferFunction(1,
                 itemsize, itemsize,
-                PyArray_DESCR(self), PyArray_DESCR(self),
+                PyArray_DESCR(self), PyArray_DESCR(mit->extra_op),
                 0, &cast_info, &transfer_flags) != NPY_SUCCEED) {
             goto finish;
         }
@@ -2035,7 +2035,6 @@ 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;
@@ -2049,7 +2048,11 @@ array_assign_subscript(PyArrayObject *self, PyObject *ind, PyObject *op)
         if (PyArray_CopyObject(tmp_arr, op) < 0) {
              goto fail;
         }
-        allocated_array = 1;
+        /*
+         * In this branch we copy directly from a newly allocated array which
+         * may have a new descr:
+         */
+        descr = PyArray_DESCR(tmp_arr);
     }
 
     if (PyArray_MapIterCheckIndices(mit) < 0) {
@@ -2097,8 +2100,7 @@ array_assign_subscript(PyArrayObject *self, PyObject *ind, PyObject *op)
         // 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),
+                descr, PyArray_DESCR(self),
                 0, &cast_info, &transfer_flags) != NPY_SUCCEED) {
             goto fail;
         }

numpy/_core/src/multiarray/nditer_constr.c

@@ -1315,8 +1315,10 @@ npyiter_check_casting(int nop, PyArrayObject **op,
         printf("\n");
 #endif
         /* If the types aren't equivalent, a cast is necessary */
-        if (op[iop] != NULL && !PyArray_EquivTypes(PyArray_DESCR(op[iop]),
-                                                     op_dtype[iop])) {
+        npy_intp view_offset = NPY_MIN_INTP;
+        if (op[iop] != NULL && !(PyArray_SafeCast(
+                    PyArray_DESCR(op[iop]), op_dtype[iop], &view_offset,
+                    NPY_NO_CASTING, 1) && view_offset == 0)) {
             /* Check read (op -> temp) casting */
             if ((op_itflags[iop] & NPY_OP_ITFLAG_READ) &&
                         !PyArray_CanCastArrayTo(op[iop],

numpy/_core/tests/test_stringdtype.py

@@ -495,15 +495,30 @@ def test_fancy_indexing(string_list):
     sarr = np.array(string_list, dtype="T")
     assert_array_equal(sarr, sarr[np.arange(sarr.shape[0])])
 
+    inds = [
+        [True, True],
+        [0, 1],
+        ...,
+        np.array([0, 1], dtype='uint8'),
+    ]
+
+    lops = [
+        ['a'*25, 'b'*25],
+        ['', ''],
+        ['hello', 'world'],
+        ['hello', 'world'*25],
+    ]
+
     # see gh-27003 and gh-27053
-    for ind in [[True, True], [0, 1], ...]:
-        for lop in [['a'*16, 'b'*16], ['', '']]:
+    for ind in inds:
+        for lop in lops:
             a = np.array(lop, dtype="T")
-            rop = ['d'*16, 'e'*16]
+            assert_array_equal(a[ind], a)
+            rop = ['d'*25, 'e'*25]
             for b in [rop, np.array(rop, dtype="T")]:
                 a[ind] = b
                 assert_array_equal(a, b)
-                assert a[0] == 'd'*16
+                assert a[0] == 'd'*25
 
 
 def test_creation_functions():