Merge pull request numpy#26066 from seberg/fix-string-sum

BUG: Allow the new string dtype summation to work

Author: ngoldbaum

numpy/_core/src/umath/ufunc_object.c

@@ -2407,6 +2407,13 @@ reducelike_promote_and_resolve(PyUFuncObject *ufunc,
                 out_descrs[0], out_descrs[1], out_descrs[2]);
         goto fail;
     }
+    /*
+     * After checking that they are equivalent, we enforce the use of the out
+     * one (which the user should have defined).  (Needed by string dtype)
+     */
+    Py_INCREF(out_descrs[2]);
+    Py_SETREF(out_descrs[0], out_descrs[2]);
+
     /* TODO: This really should _not_ be unsafe casting (same above)! */
     if (validate_casting(ufuncimpl, ufunc, ops, out_descrs, casting) < 0) {
         goto fail;

numpy/_core/tests/test_stringdtype.py

@@ -718,6 +718,18 @@ def test_ufunc_add(dtype, string_list, other_strings, use_out):
             np.add(arr1, arr2)
 
 
+def test_ufunc_add_reduce(dtype):
+    values = ["a", "this is a long string", "c"]
+    arr = np.array(values, dtype=dtype)
+    out = np.empty((), dtype=dtype)
+
+    expected = np.array("".join(values), dtype=dtype)
+    assert_array_equal(np.add.reduce(arr), expected)
+
+    np.add.reduce(arr, out=out)
+    assert_array_equal(out, expected)
+
+
 def test_add_promoter(string_list):
     arr = np.array(string_list, dtype=StringDType())
     lresult = np.array(["hello" + s for s in string_list], dtype=StringDType())