BUG: IntervalIndex.putmask with datetimelike dtypes

doc/source/whatsnew/v1.2.0.rst

@@ -587,7 +587,7 @@ Interval
 
 - Bug in :meth:`DataFrame.replace` and :meth:`Series.replace` where :class:`Interval` dtypes would be converted to object dtypes (:issue:`34871`)
 - Bug in :meth:`IntervalIndex.take` with negative indices and ``fill_value=None`` (:issue:`37330`)
--
+- Bug in :meth:`IntervalIndex.putmask` with datetime-like dtype incorrectly casting to object dtype (:issue:`37968`)
 -
 
 Indexing

pandas/core/arrays/_mixins.py

@@ -300,3 +300,24 @@ def __repr__(self) -> str:
         data = ",\n".join(lines)
         class_name = f"<{type(self).__name__}>"
         return f"{class_name}\n[\n{data}\n]\nShape: {self.shape}, dtype: {self.dtype}"
+
+    # ------------------------------------------------------------------------
+    # __array_function__ methods
+
+    def putmask(self, mask, value):
+        """
+        Analogue to np.putmask(self, mask, value)
+
+        Parameters
+        ----------
+        mask : np.ndarray[bool]
+        value : scalar or listlike
+
+        Raises
+        ------
+        TypeError
+            If value cannot be cast to self.dtype.
+        """
+        value = self._validate_setitem_value(value)
+
+        np.putmask(self._ndarray, mask, value)

pandas/core/indexes/base.py

@@ -4344,18 +4344,19 @@ def putmask(self, mask, value):
         numpy.ndarray.putmask : Changes elements of an array
             based on conditional and input values.
         """
-        values = self.values.copy()
+        values = self._values.copy()
         try:
             converted = self._validate_fill_value(value)
-            np.putmask(values, mask, converted)
-            return self._shallow_copy(values)
         except (ValueError, TypeError) as err:
             if is_object_dtype(self):
                 raise err
 
             # coerces to object
             return self.astype(object).putmask(mask, value)
 
+        np.putmask(values, mask, converted)
+        return self._shallow_copy(values)
+
     def equals(self, other: object) -> bool:
         """
         Determine if two Index object are equal.

pandas/core/indexes/extension.py

@@ -359,15 +359,13 @@ def insert(self, loc: int, item):
         return type(self)._simple_new(new_arr, name=self.name)
 
     def putmask(self, mask, value):
+        res_values = self._data.copy()
         try:
-            value = self._data._validate_setitem_value(value)
+            res_values.putmask(mask, value)
         except (TypeError, ValueError):
             return self.astype(object).putmask(mask, value)
 
-        new_values = self._data._ndarray.copy()
-        np.putmask(new_values, mask, value)
-        new_arr = self._data._from_backing_data(new_values)
-        return type(self)._simple_new(new_arr, name=self.name)
+        return type(self)._simple_new(res_values, name=self.name)
 
     def _wrap_joined_index(self: _T, joined: np.ndarray, other: _T) -> _T:
         name = get_op_result_name(self, other)

pandas/core/indexes/interval.py

@@ -858,6 +858,22 @@ def mid(self):
     def length(self):
         return Index(self._data.length, copy=False)
 
+    def putmask(self, mask, value):
+        arr = self._data.copy()
+        try:
+            value_left, value_right = arr._validate_setitem_value(value)
+        except (ValueError, TypeError):
+            return self.astype(object).putmask(mask, value)
+
+        if isinstance(self._data._left, np.ndarray):
+            np.putmask(arr._left, mask, value_left)
+            np.putmask(arr._right, mask, value_right)
+        else:
+            # TODO: special case not needed with __array_function__
+            arr._left.putmask(mask, value_left)
+            arr._right.putmask(mask, value_right)
+        return type(self)._simple_new(arr, name=self.name)
+
     @Appender(Index.where.__doc__)
     def where(self, cond, other=None):
         if other is None:

pandas/core/indexes/numeric.py

@@ -122,7 +122,7 @@ def _validate_fill_value(self, value):
             # force conversion to object
             # so we don't lose the bools
             raise TypeError
-        if isinstance(value, str):
+        elif isinstance(value, str) or lib.is_complex(value):
             raise TypeError
 
         return value

pandas/tests/indexes/interval/test_base.py

@@ -80,6 +80,30 @@ def test_where(self, closed, klass):
         result = idx.where(klass(cond))
         tm.assert_index_equal(result, expected)
 
+    @pytest.mark.parametrize("tz", ["US/Pacific", None])
+    def test_putmask_dt64(self, tz):
+        # GH#37968
+        dti = date_range("2016-01-01", periods=9, tz=tz)
+        idx = IntervalIndex.from_breaks(dti)
+        mask = np.zeros(idx.shape, dtype=bool)
+        mask[0:3] = True
+
+        result = idx.putmask(mask, idx[-1])
+        expected = IntervalIndex([idx[-1]] * 3 + list(idx[3:]))
+        tm.assert_index_equal(result, expected)
+
+    def test_putmask_td64(self):
+        # GH#37968
+        dti = date_range("2016-01-01", periods=9)
+        tdi = dti - dti[0]
+        idx = IntervalIndex.from_breaks(tdi)
+        mask = np.zeros(idx.shape, dtype=bool)
+        mask[0:3] = True
+
+        result = idx.putmask(mask, idx[-1])
+        expected = IntervalIndex([idx[-1]] * 3 + list(idx[3:]))
+        tm.assert_index_equal(result, expected)
+
     def test_getitem_2d_deprecated(self):
         # GH#30588 multi-dim indexing is deprecated, but raising is also acceptable
         idx = self.create_index()