test: pin which check decides a planner hook is a hook

9baf121 fixed a bundle setting __datafusion_session_planner__ to None -- a base
class spelling out that its subclasses contribute no planner -- being admitted
as a components bundle and then having its None called: "'NoneType' object is
not callable", naming neither the extension nor the hook. isinstance against
SessionPlannerExportable rejects an attribute bound to None; hasattr does not.

Nothing pinned it. The fix narrows the list in Python, and _commit_extensions
still carries a Rust-side hasattr as a backstop, so two checks exist and only
one decides. Handing _commit_extensions the unfiltered arguments again brings
the failure straight back, and every other test in the file passes while it
does. Verified by reverting the filter: this test is the one that fails.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: timsaucer

python/tests/test_context.py

@@ -1191,6 +1191,32 @@ def __datafusion_session_planner__(self, ctx, fallback):
     assert batches[0].column(0) == pa.array([1])
 
 
+def test_with_extensions_ignores_a_planner_attribute_set_to_none(ctx):
+    """Regression test: ``__datafusion_session_planner__ = None`` is not a hook.
+
+    A base class spelling out that its subclasses contribute no planner binds
+    the attribute to ``None`` rather than omitting it. ``isinstance`` against
+    :py:class:`~datafusion.extensions.SessionPlannerExportable` rejects that,
+    but a ``hasattr`` does not, so a host that validated with one and dispatched
+    with the other called the ``None``: ``'NoneType' object is not callable``,
+    naming neither the extension nor the hook.
+
+    ``_commit_extensions`` still carries a Rust-side ``hasattr``, and the
+    Python caller now hands it a list already narrowed by ``isinstance``. Two
+    checks means one of them decides, and this pins which: a refactor that
+    passes the unfiltered arguments through reintroduces the failure, and
+    nothing else in this file notices.
+    """
+
+    class NoPlannerExtension(_CodecOnlyExtension):
+        __datafusion_session_planner__ = None
+
+    result = ctx.with_extensions(NoPlannerExtension())
+
+    assert result.logical_extension_codec_ids() == ["my_library.logical"]
+    assert result.physical_extension_codec_ids() == ["my_library.physical"]
+
+
 def test_with_extensions_rejects_bad_codec_capsule(ctx):
     """A correctly shaped object still has to return the right capsule."""