REGR: fix eval with inplace=True to correctly update column values inplace

pandas/core/computation/eval.py

@@ -6,6 +6,8 @@
 import tokenize
 import warnings
 
+import numpy as np
+
 from pandas._libs.lib import no_default
 from pandas.util._exceptions import find_stack_level
 from pandas.util._validators import validate_bool_kwarg
@@ -384,7 +386,15 @@ def eval(
             try:
                 with warnings.catch_warnings(record=True):
                     # TODO: Filter the warnings we actually care about here.
-                    target[assigner] = ret
+                    if (
+                        inplace is True
+                        and hasattr(target, "columns")
+                        and assigner in target.columns
+                    ):
+                        loc = target.columns.get_loc(assigner)
+                        target._iset_item_mgr(loc, np.array(ret), inplace=inplace)
+                    else:
+                        target[assigner] = ret
             except (TypeError, IndexError) as err:
                 raise ValueError("Cannot assign expression output to target") from err
 

pandas/tests/computation/test_eval.py

@@ -1890,6 +1890,14 @@ def test_negate_lt_eq_le(engine, parser):
         tm.assert_frame_equal(result, expected)
 
 
+def test_set_inplace():
+    # GH 47449
+    df = DataFrame({"A": range(1, 6), "B": range(10, 0, -2), "C": range(11, 16)})
+    expected = Series([21, 20, 19, 18, 17], index=[0, 1, 2, 3, 4], name="A")
+    df.eval("A = B + C", inplace=True)
+    tm.assert_series_equal(df["A"], expected)
+
+
 class TestValidate:
     @pytest.mark.parametrize("value", [1, "True", [1, 2, 3], 5.0])
     def test_validate_bool_args(self, value):