[surviving mutant] in groupby.agg (pyarrow)

[MarcoGorelli]

If we make the following diff:

diff --git a/narwhals/_arrow/group_by.py b/narwhals/_arrow/group_by.py
index aefade0d..9445ae3b 100644
--- a/narwhals/_arrow/group_by.py
+++ b/narwhals/_arrow/group_by.py
@@ -56,7 +56,7 @@ class ArrowGroupBy:
                 in POLARS_TO_ARROW_AGGREGATIONS
             ):
                 all_simple_aggs = False
-                break
+                continue

then all tests still pass

Can we write a test which would fail if we made the above diff?

[MarcoGorelli]

Here's another one

diff --git a/narwhals/_dask/group_by.py b/narwhals/_dask/group_by.py
index f2e1ddbe..faab08c3 100644
--- a/narwhals/_dask/group_by.py
+++ b/narwhals/_dask/group_by.py
@@ -125,7 +125,7 @@ def agg_dask(
             and re.sub(r"(\w+->)", "", expr._function_name) in POLARS_TO_DASK_AGGREGATIONS
         ):
             all_simple_aggs = False
-            break
+            continue

[dangotbanned]

Can we write a test which would fail if we made the above diff?

@MarcoGorelli you probably can - it looks like you just need a simple aggregation after the the complex one.

I think both would benefit more from sharing the same implementation though.
Something like this would avoid break and continue:

class SomeGroupBy:
    _NARWHALS_TO_NATIVE_AGGREGATIONS: ClassVar[Mapping[str, Any]]

    def _ensure_all_simple(self, exprs: Sequence[CompliantExpr]):
        for expr in exprs:
            if (
                not is_elementary_expression(expr)
                and re.sub(r"(\w+->)", "", expr._function_name)
                in self._NARWHALS_TO_NATIVE_AGGREGATIONS
            ):
                msg = "..." # parameterize a format string here maybe?
                raise ValueError(msg)

    def agg(self, *exprs: CompliantExpr):
        self._ensure_all_simple(exprs)
        # <do backend specfic stuff>
        ...

Apologies if this comes across as off-topic - I keep seeing these opportunities for #1942

[MarcoGorelli]

thanks - sure, that also works (and is probably better!) 👍