MultiIndex: support isna, fixes pandas-dev#34019

Author: FRidh

pandas/core/dtypes/missing.py

@@ -153,7 +153,8 @@ def _isna(obj, inf_as_na: bool = False):
             return libmissing.checknull(obj)
     # hack (for now) because MI registers as ndarray
     elif isinstance(obj, ABCMultiIndex):
-        raise NotImplementedError("isna is not defined for MultiIndex")
+        # If there is within a level an NA, the entire label is considered NA
+        return obj.to_frame().isna().any(axis="columns").values
     elif isinstance(obj, type):
         return False
     elif isinstance(obj, (ABCSeries, np.ndarray, ABCIndex, ABCExtensionArray)):

pandas/tests/indexes/common.py

@@ -625,11 +625,6 @@ def test_nulls(self, index):
         # as these are adequately tested for function elsewhere
         if len(index) == 0:
             tm.assert_numpy_array_equal(index.isna(), np.array([], dtype=bool))
-        elif isinstance(index, MultiIndex):
-            idx = index.copy()
-            msg = "isna is not defined for MultiIndex"
-            with pytest.raises(NotImplementedError, match=msg):
-                idx.isna()
         elif not index.hasnans:
             tm.assert_numpy_array_equal(index.isna(), np.zeros(len(index), dtype=bool))
             tm.assert_numpy_array_equal(index.notna(), np.ones(len(index), dtype=bool))

pandas/tests/indexes/multi/test_missing.py

@@ -56,10 +56,7 @@ def test_dropna():
 def test_nulls(idx):
     # this is really a smoke test for the methods
     # as these are adequately tested for function elsewhere
-
-    msg = "isna is not defined for MultiIndex"
-    with pytest.raises(NotImplementedError, match=msg):
-        idx.isna()
+    idx.isna()
 
 
 @pytest.mark.xfail(reason="isna is not defined for MultiIndex")