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.

Author: camriddell

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

@@ -227,6 +227,30 @@ 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 = nw.from_native(native)
+    stablified = nw_v1.from_native(unstable)
+
+    assert isinstance(stablified, nw_v1.DataFrame)
+
+
+@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_v1 = nw_v1.from_native(native)
+    unstablified = nw.from_native(stable_v1)
+
+    assert isinstance(unstablified, nw.DataFrame)
+
+
 @pytest.mark.skipif(df_pd is None, reason="pandas not found")
 def test_series_only_dask() -> None:
     pytest.importorskip("dask")