patch: stableify concat function (#869)

* patch: stableify concat function

* @overload

* test `to_lazy`

* supposed to raise due to type mismatch

narwhals/stable/v1.py

@@ -37,7 +37,6 @@
 from narwhals.expr import Then as NwThen
 from narwhals.expr import When as NwWhen
 from narwhals.expr import when as nw_when
-from narwhals.functions import concat
 from narwhals.functions import show_versions
 from narwhals.schema import Schema as NwSchema
 from narwhals.series import Series as NwSeries
@@ -1338,6 +1337,30 @@ def mean_horizontal(*exprs: IntoExpr | Iterable[IntoExpr]) -> Expr:
     return _stableify(nw.mean_horizontal(*exprs))
 
 
+@overload
+def concat(
+    items: Iterable[DataFrame[Any]],
+    *,
+    how: Literal["horizontal", "vertical"] = "vertical",
+) -> DataFrame[Any]: ...
+
+
+@overload
+def concat(
+    items: Iterable[LazyFrame[Any]],
+    *,
+    how: Literal["horizontal", "vertical"] = "vertical",
+) -> LazyFrame[Any]: ...
+
+
+def concat(
+    items: Iterable[DataFrame[Any] | LazyFrame[Any]],
+    *,
+    how: Literal["horizontal", "vertical"] = "vertical",
+) -> DataFrame[Any] | LazyFrame[Any]:
+    return _stableify(nw.concat(items, how=how))  # type: ignore[no-any-return]
+
+
 def is_ordered_categorical(series: Series) -> bool:
     """
     Return whether indices of categories are semantically meaningful.

tests/frame/concat_test.py

@@ -10,10 +10,10 @@ def test_concat_horizontal(constructor: Any, request: Any) -> None:
     if "dask" in str(constructor):
         request.applymarker(pytest.mark.xfail)
     data = {"a": [1, 3, 2], "b": [4, 4, 6], "z": [7.0, 8, 9]}
-    df_left = nw.from_native(constructor(data))
+    df_left = nw.from_native(constructor(data)).lazy()
 
     data_right = {"c": [6, 12, -1], "d": [0, -4, 2]}
-    df_right = nw.from_native(constructor(data_right))
+    df_right = nw.from_native(constructor(data_right)).lazy()
 
     result = nw.concat([df_left, df_right], how="horizontal")
     expected = {
@@ -34,7 +34,7 @@ def test_concat_vertical(constructor: Any, request: Any) -> None:
         request.applymarker(pytest.mark.xfail)
     data = {"a": [1, 3, 2], "b": [4, 4, 6], "z": [7.0, 8, 9]}
     df_left = (
-        nw.from_native(constructor(data)).rename({"a": "c", "b": "d"}).drop("z").lazy()
+        nw.from_native(constructor(data)).lazy().rename({"a": "c", "b": "d"}).drop("z")
     )
 
     data_right = {"c": [6, 12, -1], "d": [0, -4, 2]}

tests/frame/test_invalid.py

@@ -24,7 +24,7 @@ def test_validate_laziness() -> None:
         NotImplementedError,
         match=("The items to concatenate should either all be eager, or all lazy"),
     ):
-        nw.concat([nw.from_native(df, eager_only=True), nw.from_native(df).lazy()])
+        nw.concat([nw.from_native(df, eager_only=True), nw.from_native(df).lazy()])  # type: ignore[list-item]
 
 
 @pytest.mark.skipif(