fix: Enforce no repeat aggs, fix flags

Forgot that `rust` has the `contains` check the other way

Author: dangotbanned

narwhals/_plan/aggregation.py

@@ -1,8 +1,9 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Any
 
 from narwhals._plan.common import ExprIR
+from narwhals.exceptions import InvalidOperationError
 
 if TYPE_CHECKING:
     from typing import Iterator
@@ -35,6 +36,12 @@ def iter_right(self) -> Iterator[ExprIR]:
         yield self
         yield from self.expr.iter_right()
 
+    def __init__(self, *, expr: ExprIR, **kwds: Any) -> None:
+        if expr.is_scalar:
+            msg = "Can't apply aggregations to scalar-like expressions."
+            raise InvalidOperationError(msg)
+        super().__init__(expr=expr, **kwds)  # pyright: ignore[reportCallIssue]
+
 
 class Count(Agg): ...
 

narwhals/_plan/options.py

@@ -39,10 +39,10 @@ def is_elementwise(self) -> bool:
         return self in (FunctionFlags.ROW_SEPARABLE | FunctionFlags.LENGTH_PRESERVING)
 
     def returns_scalar(self) -> bool:
-        return self in FunctionFlags.RETURNS_SCALAR
+        return FunctionFlags.RETURNS_SCALAR in self
 
     def is_length_preserving(self) -> bool:
-        return self in FunctionFlags.LENGTH_PRESERVING
+        return FunctionFlags.LENGTH_PRESERVING in self
 
     @staticmethod
     def default() -> FunctionFlags:

tests/plan/expr_parsing_test.py

@@ -13,6 +13,7 @@
 from narwhals._plan.common import ExprIR, Function
 from narwhals._plan.dummy import DummyExpr
 from narwhals._plan.expr import FunctionExpr
+from narwhals.exceptions import InvalidOperationError
 
 if TYPE_CHECKING:
     from narwhals._plan.common import IntoExpr, Seq
@@ -78,3 +79,14 @@ def test_function_expr_horizontal(
     assert isinstance(variadic_node.function, ir_node)
     assert variadic_node == sequence_node
     assert sequence_node != unrelated_node
+
+
+def test_invalid_repeat_agg() -> None:
+    with pytest.raises(InvalidOperationError):
+        nwd.col("a").mean().mean()
+    with pytest.raises(InvalidOperationError):
+        nwd.col("a").first().max()
+    with pytest.raises(InvalidOperationError):
+        nwd.col("a").any().std()
+    with pytest.raises(InvalidOperationError):
+        nwd.col("a").all().quantile(0.5, "linear")