fix: preserve old naming convention for multi-value pivot (this will …

…change in 1.0 to no longer redundantly have the column name in the middle) (#14120)

crates/polars-ops/src/frame/pivot/mod.rs

@@ -286,7 +286,8 @@ fn pivot_impl_single_column(
             let headers = column_agg.unique_stable()?.cast(&DataType::String)?;
             let mut headers = headers.str().unwrap().clone();
             if values.len() > 1 {
-                headers = headers.apply_values(|v| Cow::from(format!("{value_col_name}{sep}{v}")))
+                // TODO! MILESTONE 1.0: change to `format!("{value_col_name}{sep}{v}")`
+                headers = headers.apply_values(|v| Cow::from(format!("{value_col_name}{sep}{column}{sep}{v}")))
             }
 
             let n_cols = headers.len();

py-polars/tests/unit/operations/test_pivot.py

@@ -162,10 +162,10 @@ def test_pivot_multiple_values_column_names_5116() -> None:
     )
     expected = {
         "c1": ["A", "B"],
-        "x1|C": [1, 2],
-        "x1|D": [3, 4],
-        "x2|C": [8, 7],
-        "x2|D": [6, 5],
+        "x1|c2|C": [1, 2],
+        "x1|c2|D": [3, 4],
+        "x2|c2|C": [8, 7],
+        "x2|c2|D": [6, 5],
     }
     assert result.to_dict(as_series=False) == expected
 
@@ -188,10 +188,10 @@ def test_pivot_duplicate_names_7731() -> None:
     ).to_dict(as_series=False)
     expected = {
         "b": [1.5, 2.5],
-        'a_{"x","x"}': [1, None],
-        'a_{"x","y"}': [None, 4],
-        'd_{"x","x"}': [7, None],
-        'd_{"x","y"}': [None, 8],
+        'a_{"c","e"}_{"x","x"}': [1, None],
+        'a_{"c","e"}_{"x","y"}': [None, 4],
+        'd_{"c","e"}_{"x","x"}': [7, None],
+        'd_{"c","e"}_{"x","y"}': [None, 8],
     }
     assert result == expected