feat: Support ncs.by_name("name").meta.output_name()

Author: dangotbanned

narwhals/_plan/meta.py

@@ -12,6 +12,7 @@
 
 from narwhals._plan import expressions as ir
 from narwhals._plan._guards import is_literal
+from narwhals._plan.expressions import selectors as cs
 from narwhals._plan.expressions.literal import is_literal_scalar
 from narwhals._plan.expressions.namespace import IRNamespace
 from narwhals.exceptions import ComputeError, InvalidOperationError
@@ -143,6 +144,17 @@ def _expr_output_name(expr: ir.ExprIR, /) -> str | ComputeError:
         if isinstance(e, ir.KeepName):
             msg = "cannot determine output column without a context for this expression"
             return ComputeError(msg)
+        # https://github.com/pola-rs/polars/pull/24064
+        if isinstance(e, ir.SelectorIR):
+            # Selector with single `by_name` is fine.
+            if (
+                isinstance(e, ir.RootSelector)
+                and isinstance(e.selector, cs.ByName)
+                and len(e.selector.names) == 1
+            ):
+                return e.selector.names[0]
+            # Other selectors aren't possible right now.
+            break
         continue
     msg = (
         f"unable to find root column name for expr '{expr!r}' when calling 'output_name'"

tests/plan/meta_test.py

@@ -327,13 +327,15 @@ def test_struct_field_output_name_24003() -> None:
     assert nwp.col("ball").struct.field("radius").meta.output_name() == "radius"
 
 
-@pytest.mark.xfail(
-    reason="TODO: Single `cs.by_name` can return the name.", raises=ComputeError
-)
 def test_selector_by_name_single() -> None:
     assert ncs.by_name("foo").meta.output_name() == "foo"
 
 
 def test_selector_by_name_multiple() -> None:
-    with pytest.raises(ComputeError):
+    with pytest.raises(
+        ComputeError,
+        match=re.escape(
+            "unable to find root column name for expr 'ncs.by_name('foo', 'bar', require_all=True)' when calling 'output_name'"
+        ),
+    ):
         ncs.by_name(["foo", "bar"]).meta.output_name()