BUG: Fix Index.putmask makes stack overflow with an invalid mask

doc/source/whatsnew/v0.21.1.txt

@@ -73,7 +73,7 @@ Indexing
 - Bug in a boolean comparison of a ``datetime.datetime`` and a ``datetime64[ns]`` dtype Series (:issue:`17965`)
 - Bug where a ``MultiIndex`` with more than a million records was not raising ``AttributeError`` when trying to access a missing attribute (:issue:`18165`)
 - Bug in :class:`IntervalIndex` constructor when a list of intervals is passed with non-default ``closed`` (:issue:`18334`)
--
+- Bug in ``Index.putmask`` when an invalid mask passed (:issue:`18368`)
 -
 
 I/O

pandas/core/indexes/base.py

@@ -1939,7 +1939,10 @@ def putmask(self, mask, value):
         try:
             np.putmask(values, mask, self._convert_for_op(value))
             return self._shallow_copy(values)
-        except (ValueError, TypeError):
+        except (ValueError, TypeError) as err:
+            if is_object_dtype(self):
+                raise err
+
             # coerces to object
             return self.astype(object).putmask(mask, value)
 

pandas/tests/indexes/common.py

@@ -1032,3 +1032,16 @@ def test_map(self):
 
         dict_map = {}
         tm.assert_index_equal(index.map(dict_map), nan_index)
+
+    def test_putmask_with_wrong_mask(self):
+        # GH18368
+        index = self.create_index()
+
+        with pytest.raises(ValueError):
+            index.putmask(np.ones(len(index) + 1, np.bool), 1)
+
+        with pytest.raises(ValueError):
+            index.putmask(np.ones(len(index) - 1, np.bool), 1)
+
+        with pytest.raises(ValueError):
+            index.putmask('foo', 1)