parser: emit REFERENCES edges for Python callback arguments (#363)

[Pr1ncePS2002]

Closes #363 (Python portion).

Summary

find_dead_code already considers REFERENCES edges incoming to a node when deciding whether to flag it as dead. But for Python, those edges were never being produced for the most common callback shapes — executor.submit(handler), filter(predicate, xs), map(transform, xs), df.apply(fn), returned inner functions, etc. — so callback functions were systematically reported as unused.

Root cause

In code_review_graph/parser.py, _extract_value_references dispatches on the AST node type wrapping a call's arguments:

if node_type == \"arguments\":          # JS/TS only — Python is \"argument_list\"
    self._ref_from_arguments(...)

Python's tree-sitter grammar uses argument_list. The dispatcher never fired for Python, so _ref_from_arguments (which already knows how to walk identifier children) was never invoked.

Fix

One-line change in intent: introduce a frozenset of accepted argument-container node types and dispatch on membership.

_ARGUMENTS_TYPES = frozenset({\"arguments\", \"argument_list\"})

if node_type in self._ARGUMENTS_TYPES:
    self._ref_from_arguments(...)

Mirrors the existing _PAIR_TYPES / _ARRAY_TYPES style in the same class.

Why this doesn't add noise

_ref_from_arguments only emits when the argument is a bare identifier, and _emit_reference_if_known further gates emission on the name being locally defined or imported. So print(x) where x is a local variable does not produce a spurious REFERENCES edge to a function named x in some other file.

What this does not fix

• Keyword-argument callbacks (executor.submit(callable=h)) — keyword_argument is a different child shape; out of scope for this PR
• Function references in module-level assignments (HANDLERS = my_func) — handled by _ref_from_assignment already
• Other languages with first-class functions (Ruby Symbol#to_proc, Go function values, etc.) — separate work

Test plan

New fixture tests/fixtures/sample_callback_refs.py covers the three patterns called out in the issue body (executor.submit, filter, map). Two regression tests in tests/test_parser.py:

• test_python_callback_references_emitted — asserts REFERENCES edges land on executor_callback, filter_callback, map_callback
• test_python_callback_references_not_treated_as_dead — end-to-end check: parses the fixture, stores in a real GraphStore, runs find_dead_code, verifies the three callback functions are NOT in the dead list

Verified locally:

• uv run pytest tests/test_parser.py -k python_callback -v — 2/2 passing
• uv run pytest tests/test_parser.py -k \"value_ref or callback or reference\" -q — 13/13 passing (existing JS/TS REFERENCES tests unchanged)
• uv run ruff check code_review_graph/parser.py tests/fixtures/sample_callback_refs.py — clean
• No new failures vs main baseline (Windows-only PermissionError cluster pre-exists on both)

[Copilot]

Pull request overview

Updates the Tree-sitter parser’s “function-as-value” reference extraction so Python call argument containers (argument_list) are handled the same way as JS/TS (arguments), enabling REFERENCES edges for common callback patterns and preventing false-positive dead-code reports.

Changes:

• Expand _extract_value_references dispatch to treat both arguments and argument_list as callback-argument containers.
• Add a Python fixture demonstrating callbacks passed as bare identifiers (executor.submit / filter / map).
• Add regression tests asserting REFERENCES edges are emitted and that find_dead_code does not flag those callbacks as dead.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
code_review_graph/parser.py	Adds _ARGUMENTS_TYPES and updates value-reference dispatch so Python argument_list triggers _ref_from_arguments.
tests/fixtures/sample_callback_refs.py	New fixture covering typical Python callback-as-argument usage patterns.
tests/test_parser.py	New tests verifying REFERENCES edge emission and end-to-end dead-code behavior via GraphStore + find_dead_code.

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