fix(DRAFT): Treat polars as exception, invalidate cache

Possibly fix https://github.com/vega/altair/actions/runs/11768349827/job/32778071725?pr=3631

altair/datasets/_readers.py

@@ -99,8 +99,8 @@ def scan_fn(self, source: StrPath, /) -> Callable[..., IntoFrameT]:
         return self._scan_fn[suffix]
 
     def _response_hook(self, f):
-        # HACK: pyarrow wants the file obj
-        return f.read()
+        # HACK: `pyarrow` + `pandas` wants the file obj
+        return f
 
     def dataset(
         self,
@@ -273,6 +273,9 @@ def __init__(self, name: _Polars, /) -> None:
         }
         self._scan_fn = {".parquet": pl.scan_parquet}
 
+    def _response_hook(self, f):
+        return f.read()
+
 
 class _PolarsPyArrowReader(_Reader["pl.DataFrame", "pl.LazyFrame"]):
     def __init__(self, name: Literal["polars[pyarrow]"], /) -> None:
@@ -289,6 +292,9 @@ def __init__(self, name: Literal["polars[pyarrow]"], /) -> None:
         }
         self._scan_fn = {".parquet": pl.scan_parquet}
 
+    def _response_hook(self, f):
+        return f.read()
+
 
 class _PyArrowReader(_Reader["pa.Table", "pa.Table"]):
     """
@@ -333,9 +339,6 @@ def __init__(self, name: _PyArrow, /) -> None:
         }
         self._scan_fn = {".parquet": pa_read_parquet}
 
-    def _response_hook(self, f):
-        return f
-
 
 def _filter_reduce(predicates: tuple[Any, ...], constraints: Metadata, /) -> nw.Expr:
     """

tests/test_datasets.py

@@ -39,6 +39,7 @@ def test_loader_url(backend: _Backend) -> None:
 @backends
 def test_loader_call(backend: _Backend) -> None:
     data = Loader.with_backend(backend)
+    data.cache_dir = ""
     frame = data("stocks", ".csv")
     assert is_into_dataframe(frame)
     nw_frame = nw.from_native(frame)