throw error on empty keyed groupby (#3049)

polars/polars-core/src/frame/groupby/mod.rs

@@ -36,6 +36,12 @@ impl DataFrame {
         multithreaded: bool,
         sorted: bool,
     ) -> Result<GroupBy> {
+        if by.is_empty() {
+            return Err(PolarsError::ComputeError(
+                "expected keys in groupby operation, got nothing".into(),
+            ));
+        }
+
         macro_rules! finish_packed_bit_path {
             ($ca0:expr, $ca1:expr, $pack_fn:expr) => {{
                 let n_partitions = set_partition_size();

py-polars/tests/test_errors.py

@@ -0,0 +1,10 @@
+import pytest
+
+import polars as pl
+
+
+def test_error_on_empty_groupby() -> None:
+    with pytest.raises(
+        pl.ComputeError, match="expected keys in groupby operation, got nothing"
+    ):
+        pl.DataFrame(dict(x=[0, 0, 1, 1])).groupby([]).agg(pl.count())