fix: raise on empty by

Author: dangotbanned

narwhals/_plan/compliant/group_by.py

@@ -12,7 +12,7 @@
     FrameT_co,
     ResolverT_co,
 )
-from narwhals.exceptions import ComputeError
+from narwhals._plan.exceptions import group_by_no_keys_error
 
 if TYPE_CHECKING:
     from collections.abc import Iterator
@@ -51,8 +51,7 @@ def keys(self) -> Seq[NamedIR]:
     def key_names(self) -> Seq[str]:
         if names := self._key_names:
             return names
-        msg = "at least one key is required in a group_by operation"
-        raise ComputeError(msg)
+        raise group_by_no_keys_error()
 
 
 class EagerDataFrameGroupBy(DataFrameGroupBy[EagerDataFrameT], Protocol[EagerDataFrameT]):
@@ -163,8 +162,7 @@ def key_names(self) -> Seq[str]:
             return names
         if keys := self.keys:
             return tuple(e.name for e in keys)
-        msg = "at least one key is required in a group_by operation"
-        raise ComputeError(msg)
+        raise group_by_no_keys_error()
 
     def requires_projection(self, *, allow_aliasing: bool = False) -> bool:
         """Return True is group keys contain anything that is not a column selection.

narwhals/_plan/dataframe.py

@@ -5,6 +5,7 @@
 from narwhals._plan import _parse
 from narwhals._plan._expansion import expand_selector_irs_names, prepare_projection
 from narwhals._plan.common import ensure_seq_str, temp
+from narwhals._plan.exceptions import group_by_no_keys_error
 from narwhals._plan.group_by import GroupBy, Grouped
 from narwhals._plan.options import SortMultipleOptions
 from narwhals._plan.series import Series
@@ -266,6 +267,8 @@ def partition_by(
     ) -> list[Self]:
         by_selectors = _parse.parse_into_seq_of_selector_ir(by, *more_by)
         names = expand_selector_irs_names(by_selectors, schema=self)
+        if not names:
+            raise group_by_no_keys_error()
         partitions = self._compliant.partition_by(names, include_key=include_key)
         return [self._with_compliant(p) for p in partitions]
 

narwhals/_plan/exceptions.py

@@ -210,3 +210,8 @@ def column_index_error(
     max_nth = f"`nth({n_names - 1})`" if index >= 0 else f"`nth(-{n_names})`"
     msg = f"Invalid column index {index!r}\nHint: The schema's last column is {max_nth}"
     return ComputeError(msg)
+
+
+def group_by_no_keys_error() -> ComputeError:
+    msg = "at least one key is required in a group_by operation"
+    return ComputeError(msg)

tests/plan/frame_partition_by_test.py

@@ -129,10 +129,6 @@ def test_partition_by_missing_names(data: Data) -> None:  # pragma: no cover
         df.partition_by("c", "e")
 
 
-# TODO @dangotbanned: Stricter selectors (easier task)
-@pytest.mark.xfail(
-    reason="TODO: Guard against empty names returned from `_expansion.expand_selector_irs_names`."
-)
 def test_partition_by_fully_empty_selector(data: Data) -> None:
     df = dataframe(data)
     with pytest.raises(