BUG: Series[int].loc setitem with Series[int] results in Series[float] (#41644)

Author: simonjayhawkins

pandas/core/dtypes/cast.py

@@ -2166,6 +2166,8 @@ def can_hold_element(arr: ArrayLike, element: Any) -> bool:
     if dtype.kind in ["i", "u"]:
         if tipo is not None:
             if tipo.kind not in ["i", "u"]:
+                if is_float(element) and element.is_integer():
+                    return True
                 # Anything other than integer we cannot hold
                 return False
             elif dtype.itemsize < tipo.itemsize:

pandas/core/internals/blocks.py

@@ -781,14 +781,6 @@ def _replace_list(
             #  so un-tile here
             return self.replace(src_list, dest_list[0], inplace, regex)
 
-        # https://github.com/pandas-dev/pandas/issues/40371
-        # the following pairs check code caused a regression so we catch that case here
-        # until the issue is fixed properly in can_hold_element
-
-        # error: "Iterable[Any]" has no attribute "tolist"
-        if hasattr(src_list, "tolist"):
-            src_list = src_list.tolist()  # type: ignore[attr-defined]
-
         # Exclude anything that we know we won't contain
         pairs = [
             (x, y) for x, y in zip(src_list, dest_list) if self._can_hold_element(x)

pandas/tests/series/indexing/test_setitem.py

@@ -158,7 +158,7 @@ def test_setitem_series_object_dtype(self, indexer, ser_index):
         expected = Series([Series([42], index=[ser_index]), 0], dtype="object")
         tm.assert_series_equal(ser, expected)
 
-    @pytest.mark.parametrize("index, exp_value", [(0, 42.0), (1, np.nan)])
+    @pytest.mark.parametrize("index, exp_value", [(0, 42), (1, np.nan)])
     def test_setitem_series(self, index, exp_value):
         # GH#38303
         ser = Series([0, 0])