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

doc/source/whatsnew/v1.4.4.rst

@@ -14,8 +14,7 @@ including other versions of pandas.
 
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
--
--
+- Fixed regression in :meth:`DataFrame.eval` called with ``inplace=True`` had no effect (:issue:`47449`)
 
 .. ---------------------------------------------------------------------------
 

pandas/core/computation/eval.py

@@ -384,7 +384,10 @@ def eval(
             try:
                 with warnings.catch_warnings(record=True):
                     # TODO: Filter the warnings we actually care about here.
-                    target[assigner] = ret
+                    if hasattr(target, "loc"):
+                        target.loc[:, assigner] = ret
+                    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):