fix rename same name (#3364)

polars/polars-lazy/src/frame/mod.rs

@@ -312,7 +312,17 @@ impl LazyFrame {
         self.select_local(vec![col("*").reverse()])
     }
 
-    fn rename_impl_swapping(self, existing: Vec<String>, new: Vec<String>) -> Self {
+    fn rename_impl_swapping(self, mut existing: Vec<String>, mut new: Vec<String>) -> Self {
+        assert_eq!(new.len(), existing.len());
+        for idx in 0..existing.len() {
+            // remove "name" -> "name"
+            // these are no ops.
+            if existing[idx] == new[idx] {
+                existing.swap_remove(idx);
+                new.swap_remove(idx);
+            }
+        }
+
         // schema after renaming
         let mut new_schema = (&*self.schema()).clone();
 

py-polars/tests/test_df.py

@@ -1674,6 +1674,18 @@ def test_rename_swap() -> None:
     assert out.frame_equal(expected)
 
 
+def test_rename_same_name() -> None:
+    df = pl.DataFrame(
+        {
+            "nrs": [1, 2, 3, 4, 5],
+            "groups": ["A", "A", "B", "C", "B"],
+        }
+    ).lazy()
+    df = df.rename({"groups": "groups"})
+    df = df.select(["groups"])
+    assert df.collect().to_dict(False) == {"groups": ["A", "A", "B", "C", "B"]}
+
+
 def test_fill_null() -> None:
     df = pl.DataFrame({"a": [1, 2], "b": [3, None]})
     assert df.fill_null(4).frame_equal(pl.DataFrame({"a": [1, 2], "b": [3, 4]}))