fix: expand all literals before group_by (#11590)

crates/polars-core/src/frame/group_by/mod.rs

@@ -60,16 +60,21 @@ impl DataFrame {
             !by.is_empty(),
             ComputeError: "at least one key is required in a group_by operation"
         );
-        let by_len = by[0].len();
+        let minimal_by_len = by.iter().map(|s| s.len()).min().expect("at least 1 key");
+        let df_height = self.height();
 
         // we only throw this error if self.width > 0
         // so that we can still call this on a dummy dataframe where we provide the keys
-        if (by_len != self.height()) && (self.width() > 0) {
+        if (minimal_by_len != df_height) && (self.width() > 0) {
             polars_ensure!(
-                by_len == 1,
+                minimal_by_len == 1,
                 ShapeMismatch: "series used as keys should have the same length as the dataframe"
             );
-            by[0] = by[0].new_from_index(0, self.height())
+            for by_key in by.iter_mut() {
+                if by_key.len() == minimal_by_len {
+                    *by_key = by_key.new_from_index(0, df_height)
+                }
+            }
         };
 
         let n_partitions = _set_partition_size();

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

@@ -898,3 +898,19 @@ def test_groupby_dynamic_deprecated() -> None:
     expected = df.group_by_dynamic("date", every="2d").agg(pl.sum("value"))
     assert_frame_equal(result, expected, check_row_order=False)
     assert_frame_equal(result_lazy, expected, check_row_order=False)
+
+
+def test_group_by_multiple_keys_one_literal() -> None:
+    df = pl.DataFrame({"a": [1, 1, 2], "b": [4, 5, 6]})
+
+    expected = {"a": [1, 2], "literal": [1, 1], "b": [5, 6]}
+    for streaming in [True, False]:
+        assert (
+            df.lazy()
+            .group_by("a", pl.lit(1))
+            .agg(pl.col("b").max())
+            .sort(["a", "b"])
+            .collect(streaming=streaming)
+            .to_dict(False)
+            == expected
+        )