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fix(arrow): reject dict values that pyarrow stores as the wrong column (LAB-7490) - #459

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lab-7490-arrow-dict-value-guard
Oct 3, 2026
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lab-7490-arrow-dict-value-guard

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@27Bslash6 27Bslash6 commented Oct 2, 2026 •

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What

ArrowSerializer.serialize() and serialize_to_sink() now raise TypeError for a dict with a str, bytes, bytearray or dict value, instead of storing a wrong table without an error. Dicts of lists, NumPy arrays, pandas Series and pyarrow arrays work as before.

Why

_to_table passed any dict to pa.table, which iterates every value as a column:

s = ArrowSerializer()
d, m = s.serialize({"name": "Alice"});               s.deserialize(d, m).to_dict("list")  # {'name': ['A', 'l', 'i', 'c', 'e']}
d, m = s.serialize({"b": b"ab"});                    s.deserialize(d, m).to_dict("list")  # {'b': [97, 98]}
d, m = s.serialize({"user": {"name": "x", "age": 3}}); s.deserialize(d, m).to_dict("list")  # {'user': ['name', 'age']}

Through @cache(serializer="arrow", backend=FileBackend(...)), the second call was a hit that returned that DataFrame instead of running the function. A dict with a number, bool or None value already raised TypeError, so the validation was incomplete rather than absent.

How

  • _to_table: before pa.table(obj), an isinstance(value, (str, bytes, bytearray, Mapping)) check over the dict's values raises inside the existing try, so the existing TypeError wrapper produces the message. The message names the key and its type, for example Got a dict that is not convertible to an Arrow table: value for 'name' is str, not a list or array. Both write paths call _to_table first, so the streaming path rejects the value before it writes a byte to the sink.
  • A value that exposes an Arrow array protocol (__arrow_array__ or __arrow_c_array__) stays accepted even if it is a str, bytes or Mapping: pyarrow converts it through the protocol instead of iterating it, so it is stored as the column it describes.
  • Not changed: tuple, range and set values (stored as list columns, as today) and memoryview values (a memoryview over a typed buffer round-trips correctly today, and a byte memoryview cannot be told apart from a uint8 buffer).
  • docs/serializers/arrow.md: the "Not supported" list adds dicts with a string, bytes or dict value, the sentence that said such values are accepted but stored wrong is gone, and the type-checking example's comment quotes text the raised TypeError contains (it quoted a message the serializer never produced).
  • Entries already cached in the wrong shape stay until their TTL.

Tests

  • test_non_columnar_dict_successfully_serialized, which asserted that {"key": {"nested": "value"}} serializes, is replaced by test_non_column_dict_value_raises_type_error: str, bytes, bytearray, dict, and one bad column next to a good one, each on both paths.
  • test_dict_of_columns_round_trips_on_both_paths: list, NumPy, Series and pyarrow columns (ints and strings) round-trip, buffered and streamed.
  • test_mapping_with_arrow_array_protocol_round_trips: a Mapping adapter for each protocol round-trips on both paths. Its keys differ from its column, so iterating it instead would fail the test.
  • test_rejected_dict_writes_nothing_to_sink, and test_decorator_reruns_instead_of_returning_a_wrong_hit: through @cache on a FileBackend in tmp_path, the second call runs the function and returns the original dict.
  • The 12 new rejection, sink and decorator tests fail on main and pass here. The 4 adapter tests fail without the protocol exemption.
  • uv run pytest tests/unit/test_arrow_serializer.py: 82 passed, 2 skipped. uv run pytest tests/docs/: 70 passed. uv run pytest --markdown-docs README.md docs/: 133 passed.
  • uv run pytest tests/unit tests/critical -m "not slow": 4103 passed, 25 skipped. Serializer, encryption-composability and File backend integration tests: 38 passed. ruff check, ruff format --check and basedpyright are clean.

…n (LAB-7490)

pa.table iterates every dict value as a column, so a str value became a
column of its characters, bytes or bytearray a column of byte values, and a
dict a column of its keys with its values dropped, all without an error.
Through @cache the next call then returned that wrong DataFrame as a hit.

_to_table now raises TypeError for str, bytes, bytearray and Mapping values
before calling pa.table. Both serialize() and serialize_to_sink() go through
_to_table, so both paths reject them, and the streaming path writes nothing
to the sink. Dicts of lists, NumPy arrays, pandas Series and pyarrow arrays
are unaffected.

The test that asserted a nested dict serializes is replaced by tests for
each rejected and accepted shape on both paths, plus a decorator test on a
File backend. docs/serializers/arrow.md lists these values as unsupported
and its type-checking example quotes the message the serializer raises.
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  • docs/serializers/arrow.md

Summary by CodeRabbit

  • Bug Fixes
    • Arrow serialization now rejects dictionary values that cannot be converted into valid columns, instead of potentially interpreting them as unintended columns.
    • Failed streaming serialization no longer writes partial output, and cached functions retain their original results after a serialization failure.
  • Documentation
    • Clarified which dictionary value types are unsupported and updated the example error message.

Walkthrough

The Arrow serializer now validates dictionary column values before converting them to a table. Documentation and tests describe and verify accepted values, rejected values, streaming behaviour, and cache handling after serialization fails.

Changes

Arrow dictionary-column validation

Layer / File(s) Summary
Validate dictionary column values
src/cachekit/serializers/arrow_serializer.py, docs/serializers/arrow.md
The serializer rejects strings, bytes, bytearrays and mappings without an Arrow array or C-array protocol before table conversion. The documentation describes unsupported dictionary values and updates the error example.
Test serialization and cache behaviour
tests/unit/test_arrow_serializer.py
Tests cover round trips for supported column types, rejection of invalid values, streaming output on rejection, and cache behaviour after serialization fails.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Merge Risk: 🔵 Low · up to ea2fb

The serializer change is mergeable with a documentation correction: the current wording incorrectly rules out a supported column input.

Security Architecture Review

Security architecture risk: 🔵 Low · up to ea2fb

The change rejects inputs that previously produced incorrectly shaped cached data. Reviewed write paths contain rejection without publishing partial entries, and no new access or privilege boundary was identified. Existing incorrect cache entries are not repaired by this validation, and deployment-specific invalidation behavior remains unverified.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

  • inferred — The supported propagation scope is Arrow serialization and dependent cache writes using it. FileBackend failure containment was verified, but equivalent containment for custom streaming backends was not inspected; no broader tenant or service exposure is established by the available evidence.

Trust Boundaries and Controls

  • observed — The reviewed cache streaming path is restricted to plaintext Arrow and checks the serializer type before invocation. Dictionary validation remains inside the serializer rather than replacing tenant, encryption, or cache-dispatch policy.

Resilience and Maintainability Implications

  • observed — FileBackend’s unique temporary files and atomic publication isolate failed attempts from committed values, including exception-based interruption. Tests additionally specify an untouched direct serializer sink and repeated function execution instead of a newly malformed cache hit.
🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name Status Explanation Resolution
Docstring Coverage ⚠️ Warning Docstring coverage is 62.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 16 functions across 2 files. (1 skipped: … Write docstrings for the functions missing them to satisfy the coverage threshold.
✅ Passed checks (4 passed)
Check name Status Explanation
Title check ✅ Passed The title clearly identifies the main change: rejecting invalid dictionary values in the Arrow serializer. It is concise and specific.
Description check ✅ Passed The description is detailed and covers the change, motivation, implementation, documentation, tests, compatibility, and reported validation results. It does not use all template headings or explicitly…
Linked Issues check ✅ Passed Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check ✅ Passed Check skipped because no linked issues were found for this pull request.
Full details: Docstring Coverage

Explanation

Docstring coverage is 62.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 16 functions across 2 files. (1 skipped: 1 unsupported.)

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Files involved:
- src/cachekit/serializers/arrow_serializer.py:271

---

### [1/1] src/cachekit/serializers/arrow_serializer.py:271
Issue identified during code review:
Incomplete type check in _to_table(): the new rejection list covers str, bytes, and bytearray but omits memoryview, another bytes-like type that pyarrow iterates into a column of byte values. When a caller passes {"b": memoryview(b"ab")}, the value is silently stored as the wrong integer column [97, 98], which is the class of bug this PR is meant to stop. Fix: add memoryview to the rejected tuple, as in `(str, bytes, bytearray, memoryview, Mapping)`.
Reference implementation (from code review):

// src/cachekit/serializers/arrow_serializer.py:271
if isinstance(column, (str, bytes, bytearray, memoryview, Mapping)):

---

Review each issue in context, use the reference implementations as guidance, and apply fixes that are consistent with the surrounding codebase.

Comment thread src/cachekit/serializers/arrow_serializer.py Outdated
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kodus-27b[bot]
kodus-27b Bot previously approved these changes Oct 2, 2026
…(LAB-7490)

pa.table converts a value through __arrow_array__ or __arrow_c_array__
before it tries to iterate it, so a Mapping that implements either one is
stored as the column it describes, not as its keys. The new value guard
rejected it anyway, which broke a shape that round-tripped before.

Skip the str/bytes/bytearray/Mapping rejection when the value exposes an
Arrow array protocol. A test round-trips a Mapping adapter for each
protocol on both the buffered and streaming paths; its keys differ from its
column, so a wrong conversion would fail the test.
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Actionable comments posted: 1


  • 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @docs/serializers/arrow.md:
- Line 104: Update the accepted column types description in the Arrow serializer
documentation to describe values as Arrow-convertible columns, including
supported mappings with an Arrow array protocol and typed memoryviews. Identify
the types that are rejected, while preserving the guidance about flattening
nested dictionaries or using AutoSerializer.

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Comment thread docs/serializers/arrow.md Outdated
…-7490)

The sentence said every dict value must be a list or an array, which
excluded inputs the serializer accepts and round-trips: tuples, typed
memoryviews, and Mapping adapters that implement the Arrow array protocol.
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@27Bslash6
27Bslash6 dismissed coderabbitai[bot]’s stale review October 3, 2026 00:13

Stale: pinned to ea2fb6b, not the head; no thread is open.

@27Bslash6
27Bslash6 merged commit 5cec332 into main Oct 3, 2026
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@27Bslash6
27Bslash6 deleted the lab-7490-arrow-dict-value-guard branch October 3, 2026 11:11
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