BUG: GH10747 in astyping of a NaT-like

Author: kawochen

doc/source/whatsnew/v0.17.0.txt

@@ -629,4 +629,5 @@ Bug Fixes
 - Bug in ``pd.DataFrame`` when constructing an empty DataFrame with a string dtype (:issue:`9428`)
 - Bug in ``pd.unique`` for arrays with the ``datetime64`` or ``timedelta64`` dtype that meant an array with object dtype was returned instead the original dtype (:issue: `9431`)
 - Bug in ``DatetimeIndex.take`` and ``TimedeltaIndex.take`` may not raise ``IndexError`` against invalid index (:issue:`10295`)
+- Bug in ``Series([np.nan]).astype('M8[ms]')``, which now returns ``Series([pd.NaT])`` (:issue:`10747`) 
 - Bug in ``PeriodIndex.order`` reset freq (:issue:`10295`)

pandas/tseries/tests/test_timeseries.py

@@ -3291,6 +3291,12 @@ def test_NaT_scalar(self):
         series[2] = val
         self.assertTrue(com.isnull(series[2]))
 
+    def test_NaT_cast(self):
+        # GH10747
+        result = Series([np.nan]).astype('M8[ns]')
+        expected = Series([NaT])
+        assert_series_equal(result, expected)
+
     def test_set_none_nan(self):
         self.series[3] = None
         self.assertIs(self.series[3], NaT)

pandas/tslib.pyx

@@ -3253,9 +3253,12 @@ def cast_to_nanoseconds(ndarray arr):
 
     unit = get_datetime64_unit(arr.flat[0])
     for i in range(n):
-        pandas_datetime_to_datetimestruct(ivalues[i], unit, &dts)
-        iresult[i] = pandas_datetimestruct_to_datetime(PANDAS_FR_ns, &dts)
-        _check_dts_bounds(&dts)
+        if ivalues[i] != NPY_NAT:
+            pandas_datetime_to_datetimestruct(ivalues[i], unit, &dts)
+            iresult[i] = pandas_datetimestruct_to_datetime(PANDAS_FR_ns, &dts)
+            _check_dts_bounds(&dts)
+        else:
+            iresult[i] = NPY_NAT
 
     return result