reworked solution

pandas-dev · jreback · Aug 7, 2020 · Jun 27, 2020 · Jun 29, 2020 · Jun 27, 2020
commit 9c24cf286fd9e24374bc38f23ab91dd1d3eea8e5
diff --git a/pandas/core/groupby/groupby.py b/pandas/core/groupby/groupby.py
@@ -624,21 +624,16 @@ def get_converter(s):
             converter = get_converter(index_sample)
             names = (converter(name) for name in names)
 
-        res = [
-            [v for k, v in self.indices.items() if isna(k)]
-            if isna(name)
-            else [self.indices.get(name, [])]
-            for name in names
-        ]
-
-        return res
+        return [self.indices.get(name, []) for name in names]
 
     def _get_index(self, name):
         """
         Safe get index, translate keys for datelike to underlying repr.
         """
-        return next(iter(self._get_indices([name])), [])
-
+        if isna(name):
+            return self._get_indices([pd.NaT])[0]
+        else:
+            return self._get_indices([name])[0]
     @cache_readonly
     def _selected_obj(self):
         # Note: _selected_obj is always just `self.obj` for SeriesGroupBy

diff --git a/pandas/core/groupby/grouper.py b/pandas/core/groupby/grouper.py
@@ -561,11 +561,9 @@ def indices(self):
 
         # GH35014
         reverse_indexer = values._reverse_indexer()
-        return (
-            {**reverse_indexer, pd.NaT: [i for i, v in enumerate(values) if pd.isna(v)]}
-            if not self.dropna and any(pd.isna(v) for v in values)
-            else reverse_indexer
-        )
+        res = {**reverse_indexer, pd.NaT: np.array([i for i, v in enumerate(values) if pd.isna(v)])} if not self.dropna and any(pd.isna(v) for v in values) else reverse_indexer
+        print(f"grouper.py Grouping.indices returns {res}")
+        return res
 
     @property
     def codes(self) -> np.ndarray:

diff --git a/pandas/tests/groupby/test_groupby.py b/pandas/tests/groupby/test_groupby.py
@@ -1260,15 +1260,14 @@ def test_groupby_nat_exclude():
     }
 
     for k in grouped.indices:
-        if pd.isna(k):
-            continue  # GH 35014
         tm.assert_numpy_array_equal(grouped.indices[k], expected[k])
 
     tm.assert_frame_equal(grouped.get_group(Timestamp("2013-01-01")), df.iloc[[1, 7]])
     tm.assert_frame_equal(grouped.get_group(Timestamp("2013-02-01")), df.iloc[[3, 5]])
 
     # GH35014
-    grouped.get_group(pd.NaT)
+    with pytest.raises(KeyError):
+        grouped.get_group(pd.NaT)
 
     nan_df = DataFrame(
         {"nan": [np.nan, np.nan, np.nan], "nat": [pd.NaT, pd.NaT, pd.NaT]}