Fix NaT handling

Author: ngoldbaum

stringdtype/stringdtype/src/casts.c

@@ -1012,23 +1012,25 @@ datetime_to_string(PyArrayMethod_Context *context, char *const data[],
             /* convert to NA */
             out_ss = NULL;
         }
-        if (NpyDatetime_ConvertDatetime64ToDatetimeStruct(dt_meta, *in, &dts) <
-            0) {
-            return -1;
-        }
+        else {
+            if (NpyDatetime_ConvertDatetime64ToDatetimeStruct(dt_meta, *in,
+                                                              &dts) < 0) {
+                return -1;
+            }
 
-        // zero out buffer
-        memset(datetime_buf, 0, NPY_DATETIME_MAX_ISO8601_STRLEN);
+            // zero out buffer
+            memset(datetime_buf, 0, NPY_DATETIME_MAX_ISO8601_STRLEN);
 
-        if (NpyDatetime_MakeISO8601Datetime(
-                    &dts, datetime_buf, NPY_DATETIME_MAX_ISO8601_STRLEN, 0, 0,
-                    dt_meta->base, -1, NPY_UNSAFE_CASTING) < 0) {
-            return -1;
-        }
+            if (NpyDatetime_MakeISO8601Datetime(
+                        &dts, datetime_buf, NPY_DATETIME_MAX_ISO8601_STRLEN, 0,
+                        0, dt_meta->base, -1, NPY_UNSAFE_CASTING) < 0) {
+                return -1;
+            }
 
-        if (ssnewlen(datetime_buf, strlen(datetime_buf), out_ss) < 0) {
-            PyErr_SetString(PyExc_MemoryError, "ssnewlen failed");
-            return -1;
+            if (ssnewlen(datetime_buf, strlen(datetime_buf), out_ss) < 0) {
+                PyErr_SetString(PyExc_MemoryError, "ssnewlen failed");
+                return -1;
+            }
         }
 
         in += in_stride;

stringdtype/tests/test_stringdtype.py

@@ -523,13 +523,21 @@ def test_datetime_cast(dtype):
             np.datetime64("1923-04-14T12:43:12"),
             np.datetime64("1994-06-21T14:43:15"),
             np.datetime64("2001-10-15T04:10:32"),
+            np.datetime64("NaT"),
             np.datetime64("1995-11-25T16:02:16"),
             np.datetime64("2005-01-04T03:14:12"),
             np.datetime64("2041-12-03T14:05:03"),
         ]
     )
     sa = a.astype(dtype)
+    assert sa[3] is dtype.na_object
+
     ra = sa.astype(a.dtype)
+    assert np.isnat(ra[3])
 
     np.testing.assert_array_equal(a, ra)
+
+    # don't worry about comparing how NaT is converted
+    sa = np.delete(sa, 3)
+    a = np.delete(a, 3)
     np.testing.assert_array_equal(sa, a.astype("U"))