BUG: fix error message for multiindex.fillna (#60974)

fix error message for multiindex.fillna

Author: MarcoGorelli

Diff for: doc/source/whatsnew/v3.0.0.rst

@@ -817,6 +817,7 @@ Other
 - Bug in :meth:`DataFrame.transform` that was returning the wrong order unless the index was monotonically increasing. (:issue:`57069`)
 - Bug in :meth:`DataFrame.where` where using a non-bool type array in the function would return a ``ValueError`` instead of a ``TypeError`` (:issue:`56330`)
 - Bug in :meth:`Index.sort_values` when passing a key function that turns values into tuples, e.g. ``key=natsort.natsort_key``, would raise ``TypeError`` (:issue:`56081`)
+- Bug in :meth:`MultiIndex.fillna` error message was referring to ``isna`` instead of ``fillna`` (:issue:`60974`)
 - Bug in :meth:`Series.diff` allowing non-integer values for the ``periods`` argument. (:issue:`56607`)
 - Bug in :meth:`Series.dt` methods in :class:`ArrowDtype` that were returning incorrect values. (:issue:`57355`)
 - Bug in :meth:`Series.isin` raising ``TypeError`` when series is large (>10**6) and ``values`` contains NA (:issue:`60678`)

Diff for: pandas/core/indexes/multi.py

@@ -1760,7 +1760,7 @@ def fillna(self, value):
         """
         fillna is not implemented for MultiIndex
         """
-        raise NotImplementedError("isna is not defined for MultiIndex")
+        raise NotImplementedError("fillna is not defined for MultiIndex")
 
     @doc(Index.dropna)
     def dropna(self, how: AnyAll = "any") -> MultiIndex:

Diff for: pandas/tests/base/test_fillna.py

@@ -16,7 +16,7 @@ def test_fillna(index_or_series_obj):
     obj = index_or_series_obj
 
     if isinstance(obj, MultiIndex):
-        msg = "isna is not defined for MultiIndex"
+        msg = "fillna is not defined for MultiIndex"
         with pytest.raises(NotImplementedError, match=msg):
             obj.fillna(0)
         return

Diff for: pandas/tests/indexes/multi/test_missing.py

@@ -8,7 +8,7 @@
 
 def test_fillna(idx):
     # GH 11343
-    msg = "isna is not defined for MultiIndex"
+    msg = "fillna is not defined for MultiIndex"
     with pytest.raises(NotImplementedError, match=msg):
         idx.fillna(idx[0])
 

Diff for: pandas/tests/indexes/test_old_base.py

@@ -597,7 +597,7 @@ def test_fillna(self, index):
             pytest.skip(f"Not relevant for Index with {index.dtype}")
         elif isinstance(index, MultiIndex):
             idx = index.copy(deep=True)
-            msg = "isna is not defined for MultiIndex"
+            msg = "fillna is not defined for MultiIndex"
             with pytest.raises(NotImplementedError, match=msg):
                 idx.fillna(idx[0])
         else: