BUG: MultiIndex.putmask (#43219)

Author: jbrockmendel

doc/source/whatsnew/v1.4.0.rst

@@ -325,6 +325,7 @@ MultiIndex
 - Bug in :meth:`MultiIndex.get_loc` where the first level is a :class:`DatetimeIndex` and a string key is passed (:issue:`42465`)
 - Bug in :meth:`MultiIndex.reindex` when passing a ``level`` that corresponds to an ``ExtensionDtype`` level (:issue:`42043`)
 - Bug in :meth:`MultiIndex.get_loc` raising ``TypeError`` instead of ``KeyError`` on nested tuple (:issue:`42440`)
+- Bug in :meth:`MultiIndex.putmask` where the other value was also a :class:`MultiIndex` (:issue:`43212`)
 -
 
 I/O

pandas/core/indexes/multi.py

@@ -3683,7 +3683,12 @@ def astype(self, dtype, copy: bool = True):
         return self
 
     def _validate_fill_value(self, item):
-        if not isinstance(item, tuple):
+        if isinstance(item, MultiIndex):
+            # GH#43212
+            if item.nlevels != self.nlevels:
+                raise ValueError("Item must have length equal to number of levels.")
+            return item._values
+        elif not isinstance(item, tuple):
             # Pad the key with empty strings if lower levels of the key
             # aren't specified:
             item = (item,) + ("",) * (self.nlevels - 1)

pandas/tests/indexes/multi/test_putmask.py

@@ -0,0 +1,17 @@
+import numpy as np
+
+from pandas import MultiIndex
+import pandas._testing as tm
+
+
+def test_putmask_multiindex_other():
+    # GH#43212 `value` is also a MultiIndex
+
+    left = MultiIndex.from_tuples([(np.nan, 6), (np.nan, 6), ("a", 4)])
+    right = MultiIndex.from_tuples([("a", 1), ("a", 1), ("d", 1)])
+    mask = np.array([True, True, False])
+
+    result = left.putmask(mask, right)
+
+    expected = MultiIndex.from_tuples([right[0], right[1], left[2]])
+    tm.assert_index_equal(result, expected)