fix(serialization): find the function's symtable child by type, not by count (Python 3.14)

[Hotragn]

Rebased onto 0.8 as requested in #696. This one is a bug I hit because I moved over — it is specific to 0.8 and not present on dev.

Summary

_function_referenced_names filters a def's serialized payload down to the names its body actually needs from outside it. Its own docstring gives the reason:

a helper doing with open(path) as f: would drag in whatever f a notebook cell left lying around, and a closed file can't be pickled at all ("Cannot pickle closed files")

It located the function's symtable entry by counting children:

children = top.get_children()
if len(children) != 1:
    return _referenced_names(source)   # block rule — captures locals
return globals_of(children[0])

A single module-level def does not produce a single child table on every Python. Under PEP 649 (3.14+) it also gets an __annotate__ table — whether or not the def is annotated:

>>> import symtable
>>> [(c.get_name(), c.get_type()) for c in symtable.symtable(
...     "def f(x):\n    return g(x)\n", "<s>", "exec").get_children()]
[('__annotate__', 'annotation'), ('f', 'function')]

So on 3.14 the guard is true for every def, the fallback runs every time, and the filter is inert — reinstating exactly the capture it exists to stop. PEP 695 generics add a type_parameters table for the same reason. Note children[0] is also the wrong table by then, so dropping the guard alone would not have been enough.

Why CI didn't catch it

The two tests that already cover this — test_function_locals_are_not_shipped and test_function_globals_still_ship — fail on 3.14 today:

FAILED tests/test_serialization.py::TestScopeFiltering::test_function_locals_are_not_shipped
E   AssertionError: assert 'f' not in {'f': 'a-closed-file', 'json': 'json-mod', 'open': 'builtin'}

FAILED tests/test_serialization.py::TestScopeFiltering::test_function_globals_still_ship
E   Left contains 2 more items: {'x': 'stale', 'xs': 'stale'}

.github/workflows/python-app.yml runs Python 3.12, where PEP 649 is not active, so nothing surfaced it. pyproject.toml lists 3.14 as supported (Programming Language :: Python :: 3.14), so this is in scope rather than a future concern.

Fix

Select the one child whose get_type() is "function". That covers def and lambda, and is stable across versions — it does not care how many non-function tables a given Python decides to emit.

Tests

• test_scope_filter_survives_extra_symtable_children — parametrised over plain / annotated / defaulted def and a lambda. x is the parameter, so it is local no matter what; if it ships, the filter was bypassed. (The lambda case passes without the fix — lambdas still yield one child — which is why it is worth pinning alongside the others.)
• test_function_local_shadowing_an_unpicklable_global — the original symptom end to end: a closed file handle in the enclosing scope, shadowed by a with ... as f: local. Capturing it doesn't merely bloat the payload; serializing raises.

Result

Without the fix, 6 of 15 TestScopeFiltering tests fail on 3.14. With it, the full CPU suite is green:

856 passed, 7 skipped, 27 warnings

_{0.8 @ 1f974f0, Python 3.14.3, transformers 5.15.1, torch 2.9.1, CPU. Also confirmed the fix is a no-op on the 3.12 path (one function table either way).}

[JadenFiotto-Kaufman]

Perfect. Thank you @Hotragn