refactor: Tidy up changes from last 4 commits

- Rename and properly document "file-like object" handling
  - Also made a bit clearer what is being called and when
- Use a more granular approach to skipping in `@backends`
  - Previously, everything was skipped regardless of whether it required `pyarrow`
  - Now, `polars`, `pandas` **always** run - with `pandas` expected to fail
- I had to clean up `skip_requires_pyarrow` to make it compatible with `pytest.param`
  - It has a runtime check for if `MarkDecorator`, instead of just a callable

bb7bc17, ebc1bfa, fe0ae88,
7089f2a

altair/datasets/_readers.py

@@ -12,6 +12,7 @@
 import os
 import urllib.request
 from functools import partial
+from http.client import HTTPResponse
 from importlib import import_module
 from importlib.util import find_spec
 from itertools import chain, islice
@@ -76,6 +77,10 @@
 __all__ = ["get_backend"]
 
 
+def _identity(_: _T, /) -> _T:
+    return _
+
+
 class _Reader(Generic[IntoDataFrameT, IntoFrameT], Protocol):
     """
     Common functionality between backends.
@@ -88,6 +93,18 @@ class _Reader(Generic[IntoDataFrameT, IntoFrameT], Protocol):
     _name: LiteralString
     _ENV_VAR: ClassVar[LiteralString] = "ALTAIR_DATASETS_DIR"
     _opener: ClassVar[OpenerDirector] = urllib.request.build_opener()
+    _response: ClassVar[staticmethod[[HTTPResponse], Any]] = staticmethod(_identity)
+    """
+    Backends that do not support `file-like objects`_, must override with conversion.
+
+    Used only for **remote** files, as *cached* files use a `pathlib.Path`_.
+
+    .. _file-like objects:
+        https://docs.python.org/3/glossary.html#term-file-object
+    .. _pathlib.Path:
+        https://docs.python.org/3/library/pathlib.html#pathlib.Path
+    """
+
     _metadata: Path = Path(__file__).parent / "_metadata" / "metadata.parquet"
 
     def read_fn(self, source: StrPath, /) -> Callable[..., IntoDataFrameT]:
@@ -98,10 +115,6 @@ def scan_fn(self, source: StrPath, /) -> Callable[..., IntoFrameT]:
         suffix = validate_suffix(source, is_ext_scan)
         return self._scan_fn[suffix]
 
-    def _response_hook(self, f):
-        # HACK: `pyarrow` + `pandas` wants the file obj
-        return f
-
     def dataset(
         self,
         name: DatasetName | LiteralString,
@@ -137,7 +150,7 @@ def dataset(
                 return fn(fp, **kwds)
         else:
             with self._opener.open(url) as f:
-                return fn(self._response_hook(f), **kwds)
+                return fn(self._response(f), **kwds)
 
     def url(
         self,
@@ -261,6 +274,8 @@ def __init__(self, name: Literal["pandas[pyarrow]"], /) -> None:
 
 
 class _PolarsReader(_Reader["pl.DataFrame", "pl.LazyFrame"]):
+    _response = staticmethod(HTTPResponse.read)
+
     def __init__(self, name: _Polars, /) -> None:
         self._name = _requirements(name)
         if not TYPE_CHECKING:
@@ -273,11 +288,10 @@ 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"]):
+    _response = staticmethod(HTTPResponse.read)
+
     def __init__(self, name: Literal["polars[pyarrow]"], /) -> None:
         _pl, _pa = _requirements(name)
         self._name = name
@@ -292,9 +306,6 @@ 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"]):
     """

tests/__init__.py

@@ -5,21 +5,22 @@
 import sys
 from importlib.util import find_spec
 from pathlib import Path
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, overload
 
 import pytest
 
 from tests import examples_arguments_syntax, examples_methods_syntax
 
 if TYPE_CHECKING:
-    from collections.abc import Callable, Collection, Iterator, Mapping
+    from collections.abc import Collection, Iterator, Mapping
     from re import Pattern
 
     if sys.version_info >= (3, 11):
         from typing import TypeAlias
     else:
         from typing_extensions import TypeAlias
     from _pytest.mark import ParameterSet
+    from _pytest.mark.structures import Markable
 
     MarksType: TypeAlias = (
         "pytest.MarkDecorator | Collection[pytest.MarkDecorator | pytest.Mark]"
@@ -96,9 +97,21 @@ def windows_has_tzdata() -> bool:
 """
 
 
+@overload
 def skip_requires_pyarrow(
-    fn: Callable[..., Any] | None = None, /, *, requires_tzdata: bool = False
-) -> Callable[..., Any]:
+    fn: None = ..., /, *, requires_tzdata: bool = ...
+) -> pytest.MarkDecorator: ...
+
+
+@overload
+def skip_requires_pyarrow(
+    fn: Markable, /, *, requires_tzdata: bool = ...
+) -> Markable: ...
+
+
+def skip_requires_pyarrow(
+    fn: Markable | None = None, /, *, requires_tzdata: bool = False
+) -> pytest.MarkDecorator | Markable:
     """
     ``pytest.mark.skipif`` decorator.
 
@@ -109,7 +122,7 @@ def skip_requires_pyarrow(
     https://github.com/vega/altair/issues/3050
 
     .. _pyarrow:
-    https://pypi.org/project/pyarrow/
+        https://pypi.org/project/pyarrow/
     """
     composed = pytest.mark.skipif(
         find_spec("pyarrow") is None, reason="`pyarrow` not installed."
@@ -120,13 +133,7 @@ def skip_requires_pyarrow(
             reason="Timezone database is not installed on Windows",
         )(composed)
 
-    def wrap(test_fn: Callable[..., Any], /) -> Callable[..., Any]:
-        return composed(test_fn)
-
-    if fn is None:
-        return wrap
-    else:
-        return wrap(fn)
+    return composed if fn is None else composed(fn)
 
 
 def id_func_str_only(val) -> str:

tests/test_datasets.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import re
+from importlib.util import find_spec
 from typing import TYPE_CHECKING
 
 import pytest
@@ -14,10 +15,27 @@
 if TYPE_CHECKING:
     from altair.datasets._readers import _Backend
 
-backends = skip_requires_pyarrow(
-    pytest.mark.parametrize(
-        "backend", ["polars", "polars[pyarrow]", "pandas", "pandas[pyarrow]", "pyarrow"]
-    )
+
+requires_pyarrow = skip_requires_pyarrow()
+
+backends = pytest.mark.parametrize(
+    "backend",
+    [
+        "polars",
+        pytest.param(
+            "pandas",
+            marks=pytest.mark.xfail(
+                find_spec("pyarrow") is None,
+                reason=(
+                    "`pandas` supports backends other than `pyarrow` for `.parquet`.\n"
+                    "However, none of these are currently an `altair` dependency."
+                ),
+            ),
+        ),
+        pytest.param("polars[pyarrow]", marks=requires_pyarrow),
+        pytest.param("pandas[pyarrow]", marks=requires_pyarrow),
+        pytest.param("pyarrow", marks=requires_pyarrow),
+    ],
 )