BUG: Series[Period][mask] = 'foo' raising inconsistent with non-mask indexing

doc/source/whatsnew/v1.5.0.rst

@@ -266,6 +266,7 @@ Indexing
 - Bug in :meth:`DataFrame.iloc` where indexing a single row on a :class:`DataFrame` with a single ExtensionDtype column gave a copy instead of a view on the underlying data (:issue:`45241`)
 - Bug in setting a NA value (``None`` or ``np.nan``) into a :class:`Series` with int-based :class:`IntervalDtype` incorrectly casting to object dtype instead of a float-based :class:`IntervalDtype` (:issue:`45568`)
 - Bug in :meth:`Series.__setitem__` with a non-integer :class:`Index` when using an integer key to set a value that cannot be set inplace where a ``ValueError`` was raised insead of casting to a common dtype (:issue:`45070`)
+- Bug in :meth:`Series.__setitem__` when setting incompatible values into a ``PeriodDtype`` or ``IntervalDtype`` :class:`Series` raising when indexing with a boolean mask but coercing when indexing with otherwise-equivalent indexers; these now consistently coerce, along with :meth:`Series.mask` and :meth:`Series.where` (:issue:`45768`)
 - Bug in :meth:`Series.loc.__setitem__` and :meth:`Series.loc.__getitem__` not raising when using multiple keys without using a :class:`MultiIndex` (:issue:`13831`)
 - Bug when setting a value too large for a :class:`Series` dtype failing to coerce to a common type (:issue:`26049`, :issue:`32878`)
 - Bug in :meth:`loc.__setitem__` treating ``range`` keys as positional instead of label-based (:issue:`45479`)

pandas/core/internals/blocks.py

@@ -1380,6 +1380,8 @@ def where(self, other, cond) -> list[Block]:
 
         cond = extract_bool_array(cond)
 
+        orig_other = other
+        orig_cond = cond
         other = self._maybe_squeeze_arg(other)
         cond = self._maybe_squeeze_arg(cond)
 
@@ -1399,21 +1401,15 @@ def where(self, other, cond) -> list[Block]:
 
             if is_interval_dtype(self.dtype):
                 # TestSetitemFloatIntervalWithIntIntervalValues
-                blk = self.coerce_to_target_dtype(other)
-                if blk.dtype == _dtype_obj:
-                    # For now at least only support casting e.g.
-                    #  Interval[int64]->Interval[float64]
-                    raise
-                return blk.where(other, cond)
+                blk = self.coerce_to_target_dtype(orig_other)
+                nbs = blk.where(orig_other, orig_cond)
+                return self._maybe_downcast(nbs, "infer")
 
             elif isinstance(self, NDArrayBackedExtensionBlock):
                 # NB: not (yet) the same as
                 #  isinstance(values, NDArrayBackedExtensionArray)
-                if isinstance(self.dtype, PeriodDtype):
-                    # TODO: don't special-case
-                    raise
-                blk = self.coerce_to_target_dtype(other)
-                nbs = blk.where(other, cond)
+                blk = self.coerce_to_target_dtype(orig_other)
+                nbs = blk.where(orig_other, orig_cond)
                 return self._maybe_downcast(nbs, "infer")
 
             else:
@@ -1430,6 +1426,8 @@ def putmask(self, mask, new) -> list[Block]:
 
         values = self.values
 
+        orig_new = new
+        orig_mask = mask
         new = self._maybe_squeeze_arg(new)
         mask = self._maybe_squeeze_arg(mask)
 
@@ -1442,21 +1440,14 @@ def putmask(self, mask, new) -> list[Block]:
             if is_interval_dtype(self.dtype):
                 # Discussion about what we want to support in the general
                 #  case GH#39584
-                blk = self.coerce_to_target_dtype(new)
-                if blk.dtype == _dtype_obj:
-                    # For now at least, only support casting e.g.
-                    #  Interval[int64]->Interval[float64],
-                    raise
-                return blk.putmask(mask, new)
+                blk = self.coerce_to_target_dtype(orig_new)
+                return blk.putmask(orig_mask, orig_new)
 
             elif isinstance(self, NDArrayBackedExtensionBlock):
                 # NB: not (yet) the same as
                 #  isinstance(values, NDArrayBackedExtensionArray)
-                if isinstance(self.dtype, PeriodDtype):
-                    # TODO: don't special-case
-                    raise
-                blk = self.coerce_to_target_dtype(new)
-                return blk.putmask(mask, new)
+                blk = self.coerce_to_target_dtype(orig_new)
+                return blk.putmask(orig_mask, orig_new)
 
             else:
                 raise

pandas/tests/arrays/interval/test_interval.py

@@ -76,10 +76,16 @@ def test_set_closed(self, closed, new_closed):
         ],
     )
     def test_where_raises(self, other):
+        # GH#45768 The IntervalArray methods raises; the Series method coerces
         ser = pd.Series(IntervalArray.from_breaks([1, 2, 3, 4], closed="left"))
+        mask = np.array([True, False, True])
         match = "'value.closed' is 'right', expected 'left'."
         with pytest.raises(ValueError, match=match):
-            ser.where([True, False, True], other=other)
+            ser.array._where(mask, other)
+
+        res = ser.where(mask, other=other)
+        expected = ser.astype(object).where(mask, other)
+        tm.assert_series_equal(res, expected)
 
     def test_shift(self):
         # https://github.com/pandas-dev/pandas/issues/31495, GH#22428, GH#31502

pandas/tests/arrays/test_period.py

@@ -124,10 +124,16 @@ def test_sub_period():
     [pd.Period("2000", freq="H"), period_array(["2000", "2001", "2000"], freq="H")],
 )
 def test_where_different_freq_raises(other):
+    # GH#45768 The PeriodArray method raises, the Series method coerces
     ser = pd.Series(period_array(["2000", "2001", "2002"], freq="D"))
     cond = np.array([True, False, True])
+
     with pytest.raises(IncompatibleFrequency, match="freq"):
-        ser.where(cond, other)
+        ser.array._where(cond, other)
+
+    res = ser.where(cond, other)
+    expected = ser.astype(object).where(cond, other)
+    tm.assert_series_equal(res, expected)
 
 
 # ----------------------------------------------------------------------------

pandas/tests/frame/indexing/test_where.py

@@ -706,6 +706,20 @@ def test_where_interval_noop(self):
         res = ser.where(ser.notna())
         tm.assert_series_equal(res, ser)
 
+    def test_where_interval_fullop_downcast(self, frame_or_series):
+        # GH#45768
+        obj = frame_or_series([pd.Interval(0, 0)] * 2)
+        other = frame_or_series([1.0, 2.0])
+        res = obj.where(~obj.notna(), other)
+
+        # since all entries are being changed, we will downcast result
+        #  from object to ints (not floats)
+        tm.assert_equal(res, other.astype(np.int64))
+
+        # unlike where, Block.putmask does not downcast
+        obj.mask(obj.notna(), other, inplace=True)
+        tm.assert_equal(obj, other.astype(object))
+
     @pytest.mark.parametrize(
         "dtype",
         [
@@ -736,6 +750,16 @@ def test_where_datetimelike_noop(self, dtype):
         res4 = df.mask(mask2, "foo")
         tm.assert_frame_equal(res4, df)
 
+        # opposite case where we are replacing *all* values -> we downcast
+        #  from object dtype # GH#45768
+        res5 = df.where(mask2, 4)
+        expected = DataFrame(4, index=df.index, columns=df.columns)
+        tm.assert_frame_equal(res5, expected)
+
+        # unlike where, Block.putmask does not downcast
+        df.mask(~mask2, 4, inplace=True)
+        tm.assert_frame_equal(df, expected.astype(object))
+
 
 def test_where_try_cast_deprecated(frame_or_series):
     obj = DataFrame(np.random.randn(4, 3))
@@ -894,14 +918,29 @@ def test_where_period_invalid_na(frame_or_series, as_cat, request):
     else:
         msg = "value should be a 'Period'"
 
-    with pytest.raises(TypeError, match=msg):
-        obj.where(mask, tdnat)
+    if as_cat:
+        with pytest.raises(TypeError, match=msg):
+            obj.where(mask, tdnat)
 
-    with pytest.raises(TypeError, match=msg):
-        obj.mask(mask, tdnat)
+        with pytest.raises(TypeError, match=msg):
+            obj.mask(mask, tdnat)
+
+        with pytest.raises(TypeError, match=msg):
+            obj.mask(mask, tdnat, inplace=True)
+
+    else:
+        # With PeriodDtype, ser[i] = tdnat coerces instead of raising,
+        #  so for consistency, ser[mask] = tdnat must as well
+        expected = obj.astype(object).where(mask, tdnat)
+        result = obj.where(mask, tdnat)
+        tm.assert_equal(result, expected)
+
+        expected = obj.astype(object).mask(mask, tdnat)
+        result = obj.mask(mask, tdnat)
+        tm.assert_equal(result, expected)
 
-    with pytest.raises(TypeError, match=msg):
         obj.mask(mask, tdnat, inplace=True)
+        tm.assert_equal(obj, expected)
 
 
 def test_where_nullable_invalid_na(frame_or_series, any_numeric_ea_dtype):

pandas/tests/series/indexing/test_setitem.py

@@ -18,6 +18,7 @@
     IntervalIndex,
     MultiIndex,
     NaT,
+    Period,
     Series,
     Timedelta,
     Timestamp,
@@ -1275,6 +1276,22 @@ def obj(self):
         return Series(timedelta_range("1 day", periods=4))
 
 
+@pytest.mark.parametrize(
+    "val", ["foo", Period("2016", freq="Y"), Interval(1, 2, closed="both")]
+)
+@pytest.mark.parametrize("exp_dtype", [object])
+class TestPeriodIntervalCoercion(CoercionTest):
+    # GH#45768
+    @pytest.fixture(
+        params=[
+            period_range("2016-01-01", periods=3, freq="D"),
+            interval_range(1, 5),
+        ]
+    )
+    def obj(self, request):
+        return Series(request.param)
+
+
 def test_20643():
     # closed by GH#45121
     orig = Series([0, 1, 2], index=["a", "b", "c"])