Backport PR #42166: REGR: preserve Int32 dtype on setitem (#42172)

Co-authored-by: jbrockmendel <[email protected]>

Author: meeseeksmachine

pandas/core/indexing.py

@@ -42,8 +42,12 @@
     isna,
 )
 
+from pandas.core import algorithms as algos
 import pandas.core.common as com
-from pandas.core.construction import array as pd_array
+from pandas.core.construction import (
+    array as pd_array,
+    extract_array,
+)
 from pandas.core.indexers import (
     check_array_indexer,
     is_empty_indexer,
@@ -1661,6 +1665,21 @@ def _setitem_with_indexer(self, indexer, value, name="iloc"):
                         if com.is_null_slice(indexer[0]):
                             # We are setting an entire column
                             self.obj[key] = value
+                            return
+                        elif is_array_like(value):
+                            # GH#42099
+                            arr = extract_array(value, extract_numpy=True)
+                            taker = -1 * np.ones(len(self.obj), dtype=np.intp)
+                            empty_value = algos.take_nd(arr, taker)
+                            if not isinstance(value, ABCSeries):
+                                # if not Series (in which case we need to align),
+                                #  we can short-circuit
+                                empty_value[indexer[0]] = arr
+                                self.obj[key] = empty_value
+                                return
+
+                            self.obj[key] = empty_value
+
                         else:
                             self.obj[key] = infer_fill_value(value)
 

pandas/tests/indexing/test_loc.py

@@ -1830,6 +1830,23 @@ def test_loc_setitem_with_expansion_nonunique_index(self, index, request):
         )
         tm.assert_frame_equal(df, expected)
 
+    @pytest.mark.parametrize(
+        "dtype", ["Int32", "Int64", "UInt32", "UInt64", "Float32", "Float64"]
+    )
+    def test_loc_setitem_with_expansion_preserves_nullable_int(self, dtype):
+        # GH#42099
+        ser = Series([0, 1, 2, 3], dtype=dtype)
+        df = DataFrame({"data": ser})
+
+        result = DataFrame(index=df.index)
+        result.loc[df.index, "data"] = ser
+
+        tm.assert_frame_equal(result, df)
+
+        result = DataFrame(index=df.index)
+        result.loc[df.index, "data"] = ser._values
+        tm.assert_frame_equal(result, df)
+
 
 class TestLocCallable:
     def test_frame_loc_getitem_callable(self):