fix: Update version when passing narwhals objects in from_native (#3515)

* add v2 to stable/__init__:__all__

* ref: test_init_already_narwhals duplicate test

in tests/translate/from_native_test.py
test_init_already_narwhals and test_init_already_narwhals_unstable
have the exact same source code, when the former should verify behavior
in a stable version.

This function used to test this, but seemed to have been mistakenly
edited.

* fix: from_native updates a passed nw.DataFrame with the passed version

from_native on a narwhals.{DataFrame,Series} would always return
the same object even if the DataFrame._version and the version
passed into from_native disagreed.

Now, if the versions mismatch we create a new narwhals object
with the appropriate version. If the versions match, we short cut
and return the same object.

* fix nw.LazyFrame missing _version ClassVar

* add Series objects in to_stable/to_unstable tests

* ignore var-annotated in tests/translate/from_native init_already to {stable,unstable}

Author: camriddell

narwhals/dataframe.py

@@ -2361,6 +2361,8 @@ class LazyFrame(BaseFrame[LazyFrameT]):
         ```
     """
 
+    _version: ClassVar[Version] = Version.MAIN
+
     @property
     def _compliant(self) -> CompliantLazyFrame[Any, LazyFrameT, Self]:
         return self._compliant_frame

narwhals/stable/__init__.py

@@ -1,5 +1,5 @@
 from __future__ import annotations
 
-from narwhals.stable import v1
+from narwhals.stable import v1, v2
 
-__all__ = ["v1"]
+__all__ = ["v1", "v2"]

narwhals/stable/v1/__init__.py

@@ -258,6 +258,8 @@ def _l1_norm(self) -> Self:
 
 
 class LazyFrame(NwLazyFrame[IntoLazyFrameT]):
+    _version = Version.V1
+
     @inherit_doc(NwLazyFrame)
     def __init__(self, df: Any, *, level: Literal["full", "lazy", "interchange"]) -> None:
         assert df._version is Version.V1  # noqa: S101

narwhals/stable/v2/__init__.py

@@ -241,6 +241,8 @@ def is_unique(self) -> Series[Any]:
 
 
 class LazyFrame(NwLazyFrame[IntoLazyFrameT]):
+    _version = Version.V2
+
     @inherit_doc(NwLazyFrame)
     def __init__(self, df: Any, *, level: Literal["full", "lazy", "interchange"]) -> None:
         assert df._version is Version.V2  # noqa: S101

narwhals/translate.py

@@ -282,9 +282,25 @@ def _from_native_impl(  # noqa: C901, PLR0911, PLR0912, PLR0915
 
     # Early returns
     if isinstance(native_object, (DataFrame, LazyFrame)) and not series_only:
-        return native_object
+        if native_object._version is version:
+            return native_object
+
+        real_native_object = native_object.to_native()
+        return (
+            version.namespace.from_native_object(real_native_object)
+            .compliant.from_native(real_native_object)
+            .to_narwhals()
+        )
     if isinstance(native_object, Series) and (series_only or allow_series):
-        return native_object
+        if native_object._version is version:
+            return native_object
+
+        real_native_object = native_object.to_native()
+        return (
+            version.namespace.from_native_object(real_native_object)
+            .compliant.from_native(real_native_object)
+            .to_narwhals()
+        )
 
     if series_only:
         if allow_series is False:

tests/translate/from_native_test.py

@@ -206,12 +206,14 @@ def test_pandas_like_validate() -> None:
 
 
 @pytest.mark.skipif(lf_pl is None, reason="polars not found")
-def test_init_already_narwhals() -> None:
-    df = nw.from_native(pl.DataFrame({"a": [1, 2, 3]}))
-    result = nw.from_native(df)
+def test_init_already_narwhals_stable() -> None:
+    from narwhals.stable import v1 as nw_v1
+
+    df = nw_v1.from_native(pl.DataFrame({"a": [1, 2, 3]}))
+    result = nw_v1.from_native(df)
     assert result is df
     s = df["a"]
-    result_s = nw.from_native(s, allow_series=True)
+    result_s = nw_v1.from_native(s, allow_series=True)
     assert result_s is s
 
 
@@ -225,6 +227,38 @@ def test_init_already_narwhals_unstable() -> None:
     assert result_s is s
 
 
+@pytest.mark.skipif(lf_pl is None, reason="polars not found")
+def test_init_already_narwhals_unstable_to_stable() -> None:
+    from narwhals.stable import v1 as nw_v1
+
+    native = pl.DataFrame({"a": [1, 2, 3]})
+
+    unstable_df = nw.from_native(native)
+    stablified_df = nw_v1.from_native(unstable_df)
+    assert isinstance(stablified_df, nw_v1.DataFrame)
+
+    s = native["a"]
+    unstable_s = nw.from_native(s, allow_series=True)
+    stablified_s = nw_v1.from_native(unstable_s, allow_series=True)  # type: ignore[var-annotated]
+    assert isinstance(stablified_s, nw_v1.Series)
+
+
+@pytest.mark.skipif(lf_pl is None, reason="polars not found")
+def test_init_already_narwhals_stable_to_unstable() -> None:
+    from narwhals.stable import v1 as nw_v1
+
+    native = pl.DataFrame({"a": [1, 2, 3]})
+
+    stable_df = nw_v1.from_native(native)
+    unstablified_df = nw.from_native(stable_df)
+    assert isinstance(unstablified_df, nw.DataFrame)
+
+    s = native["a"]
+    stable_s = nw_v1.from_native(s, allow_series=True)  # type: ignore[var-annotated]
+    unstablified_s = nw.from_native(stable_s, allow_series=True)  # type: ignore[var-annotated]
+    assert isinstance(unstablified_s, nw.Series)
+
+
 @pytest.mark.skipif(df_pd is None, reason="pandas not found")
 def test_series_only_dask() -> None:
     pytest.importorskip("dask")

tests/v1_test.py

@@ -621,7 +621,7 @@ def test_lazyframe_recursive_v1() -> None:
 
     pl_frame = pl.DataFrame({"a": [1, 2, 3]}).lazy()
     nw_frame = nw_v1.from_native(pl_frame)
-    with pytest.raises(AttributeError):
+    with pytest.raises(AssertionError):
         nw_v1.LazyFrame(nw_frame, level="lazy")
 
     nw_frame_early_return = nw_v1.from_native(nw_frame)