REF: Change _NULL_DESCRIPTION[datetime] to use NaT sentinel (pandas-d…

…ev#47887)

* REF: Change _NULL_DESCRIPTION[datetime] to use NaT sentinel

* Add test

* Add roundtrip test and change to use iNaT

pandas/core/interchange/column.py

@@ -5,6 +5,7 @@
 import numpy as np
 
 from pandas._libs.lib import infer_dtype
+from pandas._libs.tslibs import iNaT
 from pandas.util._decorators import cache_readonly
 
 import pandas as pd
@@ -38,7 +39,7 @@
 
 _NULL_DESCRIPTION = {
     DtypeKind.FLOAT: (ColumnNullType.USE_NAN, None),
-    DtypeKind.DATETIME: (ColumnNullType.USE_NAN, None),
+    DtypeKind.DATETIME: (ColumnNullType.USE_SENTINEL, iNaT),
     DtypeKind.INT: (ColumnNullType.NON_NULLABLE, None),
     DtypeKind.UINT: (ColumnNullType.NON_NULLABLE, None),
     DtypeKind.BOOL: (ColumnNullType.NON_NULLABLE, None),

pandas/core/interchange/dataframe_protocol.py

@@ -70,7 +70,7 @@ class ColumnNullType(enum.IntEnum):
     NON_NULLABLE : int
         Non-nullable column.
     USE_NAN : int
-        Use explicit float NaN/NaT value.
+        Use explicit float NaN value.
     USE_SENTINEL : int
         Sentinel value besides NaN/NaT.
     USE_BITMASK : int

pandas/tests/interchange/test_impl.py

@@ -4,6 +4,8 @@
 import numpy as np
 import pytest
 
+from pandas._libs.tslibs import iNaT
+
 import pandas as pd
 import pandas._testing as tm
 from pandas.core.interchange.dataframe_protocol import (
@@ -176,3 +178,15 @@ def test_nonstring_object():
     col = df.__dataframe__().get_column_by_name("A")
     with pytest.raises(NotImplementedError, match="not supported yet"):
         col.dtype
+
+
+def test_datetime():
+    df = pd.DataFrame({"A": [pd.Timestamp("2022-01-01"), pd.NaT]})
+    col = df.__dataframe__().get_column_by_name("A")
+
+    assert col.size == 2
+    assert col.null_count == 1
+    assert col.dtype[0] == DtypeKind.DATETIME
+    assert col.describe_null == (ColumnNullType.USE_SENTINEL, iNaT)
+
+    tm.assert_frame_equal(df, from_dataframe(df.__dataframe__()))