BUG: broadcasting listlike values in Series.__setitem__ GH#44265

doc/source/whatsnew/v1.4.0.rst

@@ -540,6 +540,7 @@ Indexing
 - Bug in setting a scalar :class:`Interval` value into a :class:`Series` with ``IntervalDtype`` when the scalar's sides are floats and the values' sides are integers (:issue:`44201`)
 - Bug when setting string-backed :class:`Categorical` values that can be parsed to datetimes into a :class:`DatetimeArray` or :class:`Series` or :class:`DataFrame` column backed by :class:`DatetimeArray` failing to parse these strings (:issue:`44236`)
 - Bug in :meth:`Series.__setitem__` with an integer dtype other than ``int64`` setting with a ``range`` object unnecessarily upcasting to ``int64`` (:issue:`44261`)
+- Bug in :meth:`Series.__setitem__` with a boolean mask indexer setting a listlike value of length 1 incorrectly broadcasting that value (:issue:`44265`)
 -
 
 Missing

pandas/core/series.py

@@ -1096,9 +1096,26 @@ def __setitem__(self, key, value) -> None:
             if com.is_bool_indexer(key):
                 key = check_bool_indexer(self.index, key)
                 key = np.asarray(key, dtype=bool)
+
+                if (
+                    is_list_like(value)
+                    and len(value) != len(self)
+                    and not isinstance(value, Series)
+                    and not is_object_dtype(self.dtype)
+                ):
+                    # Series will be reindexed to have matching length inside
+                    #  _where call below
+                    # GH#44265
+                    indexer = key.nonzero()[0]
+                    self._set_values(indexer, value)
+                    return
+
+                # otherwise with listlike other we interpret series[mask] = other
+                #  as series[mask] = other[mask]
                 try:
                     self._where(~key, value, inplace=True)
                 except InvalidIndexError:
+                    # test_where_dups
                     self.iloc[key] = value
                 return
 

pandas/tests/series/indexing/test_setitem.py

@@ -1064,3 +1064,43 @@ def test_setitem_with_bool_indexer():
     df.loc[[True, False, False], "a"] = 10
     expected = DataFrame({"a": [10, 2, 3]})
     tm.assert_frame_equal(df, expected)
+
+
+@pytest.mark.parametrize("size", range(2, 6))
+@pytest.mark.parametrize(
+    "mask", [[True, False, False, False, False], [True, False], [False]]
+)
+@pytest.mark.parametrize(
+    "item", [2.0, np.nan, np.finfo(float).max, np.finfo(float).min]
+)
+# Test numpy arrays, lists and tuples as the input to be
+# broadcast
+@pytest.mark.parametrize(
+    "box", [lambda x: np.array([x]), lambda x: [x], lambda x: (x,)]
+)
+def test_setitem_bool_indexer_dont_broadcast_length1_values(size, mask, item, box):
+    # GH#44265
+    # see also tests.series.indexing.test_where.test_broadcast
+
+    selection = np.resize(mask, size)
+
+    data = np.arange(size, dtype=float)
+
+    ser = Series(data)
+
+    if selection.sum() != 1:
+        msg = (
+            "cannot set using a list-like indexer with a different "
+            "length than the value"
+        )
+        with pytest.raises(ValueError, match=msg):
+            # GH#44265
+            ser[selection] = box(item)
+    else:
+        # In this corner case setting is equivalent to setting with the unboxed
+        #  item
+        ser[selection] = box(item)
+
+        expected = Series(np.arange(size, dtype=float))
+        expected[selection] = item
+        tm.assert_series_equal(ser, expected)

pandas/tests/series/indexing/test_where.py

@@ -88,7 +88,7 @@ def test_where_unsafe():
     s = Series(np.arange(10))
     mask = s > 5
 
-    msg = "cannot assign mismatch length to masked array"
+    msg = "cannot set using a list-like indexer with a different length than the value"
     with pytest.raises(ValueError, match=msg):
         s[mask] = [5, 4, 3, 2, 1]
 
@@ -161,13 +161,10 @@ def test_where_error():
     tm.assert_series_equal(s, expected)
 
     # failures
-    msg = "cannot assign mismatch length to masked array"
+    msg = "cannot set using a list-like indexer with a different length than the value"
     with pytest.raises(ValueError, match=msg):
         s[[True, False]] = [0, 2, 3]
-    msg = (
-        "NumPy boolean array indexing assignment cannot assign 0 input "
-        "values to the 1 output values where the mask is true"
-    )
+
     with pytest.raises(ValueError, match=msg):
         s[[True, False]] = []
 
@@ -298,6 +295,7 @@ def test_where_setitem_invalid():
     "box", [lambda x: np.array([x]), lambda x: [x], lambda x: (x,)]
 )
 def test_broadcast(size, mask, item, box):
+    # GH#8801, GH#4195
     selection = np.resize(mask, size)
 
     data = np.arange(size, dtype=float)
@@ -309,7 +307,8 @@ def test_broadcast(size, mask, item, box):
     )
 
     s = Series(data)
-    s[selection] = box(item)
+
+    s[selection] = item
     tm.assert_series_equal(s, expected)
 
     s = Series(data)