Fix MultiIndex .loc "Too Many Indexers" with None as return value (#34450)

Author: pedrooa

doc/source/whatsnew/v1.1.0.rst

@@ -887,6 +887,7 @@ Indexing
 - Bug in :meth:`DataFrame.truncate` and :meth:`Series.truncate` where index was assumed to be monotone increasing (:issue:`33756`)
 - Indexing with a list of strings representing datetimes failed on :class:`DatetimeIndex` or :class:`PeriodIndex`(:issue:`11278`)
 - Bug in :meth:`Series.at` when used with a :class:`MultiIndex` would raise an exception on valid inputs (:issue:`26989`)
+- Bug in :meth:`Series.loc` when used with a :class:`MultiIndex` would raise an IndexingError when accessing a None value (:issue:`34318`)
 
 Missing
 ^^^^^^^

pandas/core/indexing.py

@@ -768,9 +768,11 @@ def _getitem_lowerdim(self, tup: Tuple):
         # ...but iloc should handle the tuple as simple integer-location
         # instead of checking it as multiindex representation (GH 13797)
         if isinstance(ax0, ABCMultiIndex) and self.name != "iloc":
-            result = self._handle_lowerdim_multi_index_axis0(tup)
-            if result is not None:
+            try:
+                result = self._handle_lowerdim_multi_index_axis0(tup)
                 return result
+            except IndexingError:
+                pass
 
         if len(tup) > self.ndim:
             raise IndexingError("Too many indexers. handle elsewhere")
@@ -818,9 +820,11 @@ def _getitem_nested_tuple(self, tup: Tuple):
             if self.name != "loc":
                 # This should never be reached, but lets be explicit about it
                 raise ValueError("Too many indices")
-            result = self._handle_lowerdim_multi_index_axis0(tup)
-            if result is not None:
+            try:
+                result = self._handle_lowerdim_multi_index_axis0(tup)
                 return result
+            except IndexingError:
+                pass
 
             # this is a series with a multi-index specified a tuple of
             # selectors
@@ -1067,7 +1071,7 @@ def _handle_lowerdim_multi_index_axis0(self, tup: Tuple):
             if len(tup) <= self.obj.index.nlevels and len(tup) > self.ndim:
                 raise ek
 
-        return None
+        raise IndexingError("No label returned")
 
     def _getitem_axis(self, key, axis: int):
         key = item_from_zerodim(key)

pandas/tests/series/indexing/test_multiindex.py

@@ -0,0 +1,22 @@
+""" test get/set & misc """
+
+
+import pandas as pd
+from pandas import MultiIndex, Series
+
+
+def test_access_none_value_in_multiindex():
+    # GH34318: test that you can access a None value using .loc through a Multiindex
+
+    s = Series([None], pd.MultiIndex.from_arrays([["Level1"], ["Level2"]]))
+    result = s.loc[("Level1", "Level2")]
+    assert result is None
+
+    midx = MultiIndex.from_product([["Level1"], ["Level2_a", "Level2_b"]])
+    s = Series([None] * len(midx), dtype=object, index=midx)
+    result = s.loc[("Level1", "Level2_a")]
+    assert result is None
+
+    s = Series([1] * len(midx), dtype=object, index=midx)
+    result = s.loc[("Level1", "Level2_a")]
+    assert result == 1