Merge pull request numpy#28228 from ngoldbaum/float-string-nan-cast

BUG: handle case when StringDType na_object is nan in float to string cast

numpy/_core/src/multiarray/stringdtype/casts.cpp

@@ -1208,14 +1208,35 @@ float_to_string(
     PyArray_StringDTypeObject *descr =
             (PyArray_StringDTypeObject *)context->descriptors[1];
     npy_string_allocator *allocator = NpyString_acquire_allocator(descr);
+    // borrowed reference
+    PyObject *na_object = descr->na_object;
 
     while (N--) {
         PyObject *scalar_val = PyArray_Scalar(in, float_descr, NULL);
+        if (descr->has_nan_na) {
+            // check for case when scalar_val is the na_object and store a null string
+            int na_cmp = na_eq_cmp(scalar_val, na_object);
+            if (na_cmp < 0) {
+                Py_DECREF(scalar_val);
+                goto fail;
+            }
+            if (na_cmp) {
+                Py_DECREF(scalar_val);
+                if (NpyString_pack_null(allocator, (npy_packed_static_string *)out) < 0) {
+                    PyErr_SetString(PyExc_MemoryError,
+                                    "Failed to pack null string during float "
+                                    "to string cast");
+                    goto fail;
+                }
+                goto next_step;
+            }
+        }
         // steals reference to scalar_val
         if (pyobj_to_string(scalar_val, out, allocator) == -1) {
             goto fail;
         }
 
+      next_step:
         in += in_stride;
         out += out_stride;
     }

numpy/_core/src/multiarray/stringdtype/dtype.c

@@ -144,7 +144,7 @@ new_stringdtype_instance(PyObject *na_object, int coerce)
     return NULL;
 }
 
-static int
+NPY_NO_EXPORT int
 na_eq_cmp(PyObject *a, PyObject *b) {
     if (a == b) {
         // catches None and other singletons like Pandas.NA

numpy/_core/src/multiarray/stringdtype/dtype.h

@@ -52,6 +52,9 @@ _eq_comparison(int scoerce, int ocoerce, PyObject *sna, PyObject *ona);
 NPY_NO_EXPORT int
 stringdtype_compatible_na(PyObject *na1, PyObject *na2, PyObject **out_na);
 
+NPY_NO_EXPORT int
+na_eq_cmp(PyObject *a, PyObject *b);
+
 #ifdef __cplusplus
 }
 #endif

numpy/_core/tests/test_stringdtype.py

@@ -735,6 +735,21 @@ def test_float_casts(typename):
     assert_array_equal(eres, res)
 
 
+def test_float_nan_cast_na_object():
+    # gh-28157
+    dt = np.dtypes.StringDType(na_object=np.nan)
+    arr1 = np.full((1,), fill_value=np.nan, dtype=dt)
+    arr2 = np.full_like(arr1, fill_value=np.nan)
+
+    assert arr1.item() is np.nan
+    assert arr2.item() is np.nan
+
+    inp = [1.2, 2.3, np.nan]
+    arr = np.array(inp).astype(dt)
+    assert arr[2] is np.nan
+    assert arr[0] == '1.2'
+
+
 @pytest.mark.parametrize(
     "typename",
     [