fix for df.eval can't concatenate string column and string via +

Author: dannyi96

pandas/core/computation/ops.py

@@ -238,7 +238,7 @@ def return_type(self):
     @property
     def has_invalid_return_type(self) -> bool:
         types = self.operand_types
-        obj_dtype_set = frozenset([np.dtype("object")])
+        obj_dtype_set = frozenset([np.dtype("object"), str])
         return self.return_type == object and types - obj_dtype_set
 
     @property

pandas/tests/computation/test_eval.py

@@ -743,6 +743,13 @@ def test_eval_keep_name(self, engine, parser):
         expected = Series([4, 30, 56], name="a")
         tm.assert_series_equal(expected, res)
 
+    def test_obj_str_addition(self, engine, parser):
+        # GH 47734
+        df = DataFrame({"a": ["a", "b"]})
+        res = df.eval("a + 'a'", engine="python")
+        expected = Series(["aa", "ba"], name="a")
+        tm.assert_series_equal(expected, res)
+
     def test_eval_unmatching_names(self, engine, parser):
         variable_name = Series([42], name="series_name")
         res = pd.eval("variable_name + 0", engine=engine, parser=parser)