fix(python): honor custom Equals and GetHashCode in structural values

[dbrattli]

Fixes #4834

Problem

An F# class can override Equals and GetHashCode without opting into Python's __eq__ and __hash__ protocols. The Python runtime checked the Python protocols first, so such a class reached the inherited object.__hash__ (identity hash), and that identity hash leaked into unions, records and arrays. Union equality was also field-wise Python ==.

Running the reproduction from the issue on main:

class equality: true
class hash: false     <- expected true
union equality: false <- expected true
union hash: false     <- expected true
record equality: true
record hash: false    <- expected true
array equality: true
array hash: false     <- expected true

All eight now print true, matching .NET and the JS/TS targets.

Changes

This is solution 1 from the issue — correct the existing helpers, so already-generated Python code is fixed by upgrading fable-library. No compiler changes. The Python runtime now mirrors the TypeScript one.

fable_library/util.py

• safe_hash delegates to identity_hash, so F# GetHashCode wins over Python __hash__ (it was the other way round).
• structural_hash was int32(hash(x)). It now dispatches like TS structuralHash: None/bool/str, then GetHashCode, then numbers, then element-wise array_hash for Array/tuple/list, then Python hash, then identity. array_hash is fixed by the same change. As a side effect, structural_hash on a Python list no longer raises TypeError.

fable_library/union.py

• Union.Equals compares fields with equal_arrays (F# equals) instead of Python ==.
• Union.GetHashCode combines number_hash(tag) with structural_hash per field instead of hashing a Python tuple.
• @tagged_union builds the dataclass with eq=False, so case classes inherit F# equality rather than dataclass field-wise ==.

fable_library/record.py

• record_get_hashcode combines per-field structural_hash instead of hashing a Python tuple.

Incidental fix

EquatableBase declares __eq__, so Python implicitly sets EquatableBase.__hash__ = None, and that None shadows HashableBase.__hash__ in the MRO. #4805 worked around this inside @tagged_union; with eq=False that workaround no longer applies, so __hash__ is now defined on Union and Record directly. This also fixes records: 9 of the 106 generated record classes in the Python test output have no emitted __hash__ and were unhashable from Python.

Tests

Added Custom Equals and GetHashCode are used by structural values to tests/Python/TestComparison.fs and tests/Js/Main/ComparisonTests.fs. JS/TS already behaved correctly, so that one is a regression guard.

• Python: 2417 passed (was 2416). Pyright unchanged at 34 errors / 45 warnings, and clean on the three modified library files. ruff format --check clean.
• JavaScript: 3081 passed. TypeScript: 2860 passed.

Performance

Measured on the path Fable actually generates for union equality (util.equals), 200k iterations:

	before	after
equals(a, b)	1.45 µs	1.29 µs
safe_hash(a)	0.81 µs	1.28 µs
Python a == b	0.06 µs	1.19 µs

The compiled path is slightly faster. Python-level == on a union case is slower because dataclass tuple comparison is replaced by F# equality — that is the point of the fix.

🤖 Generated with Claude Code

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	34
Files with errors	4
Excluded files	4
New errors	✅ No

Excluded files with errors (4 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_hash_set.py	18	Excluded
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	2	Excluded