fix(python): make [<AttachMembers>] union static members work (#4634)

[dbrattli]

Fixes #4634

Problem

A discriminated union with [<AttachMembers>] is emitted in Python as three pieces:

• a private base class _Demo(Union) that holds the attached static members,
• the case classes Demo_A, Demo_B,
• a public type alias type Demo = Demo_A | Demo_B.

Given:

[<AttachMembers>]
type Demo =
    | A of string
    | B
    static member propDefault = A "prop"
    static member methDefault() = A "meth"

this shape was broken in two ways (plus a latent third):

1. Eager evaluation, raising NameError at import. The static property was emitted as a class-body assignment inside _Demo:

   prop_default = StaticProperty["Demo"](Demo_A("prop"))

   Python evaluates Demo_A("prop") while defining _Demo, before the case class exists, so the module failed to import.

2. Use sites hit the type alias. Demo.propDefault / Demo.methDefault() compiled to Demo.prop_default / Demo.meth_default(), but Demo is a TypeAliasType that does not carry the members, raising AttributeError. This broke both static methods and static properties.

3. (Latent) bad type annotation. When the union lived in a module, the descriptor annotation was emitted as a bare StaticLazyProperty[Issue4634_Demo] instead of a string forward reference, also raising NameError. The eager-evaluation bug masked it.

Fix

• getInitialValue / transformStaticProperty (Fable2Python.Transforms.fs): defer a read-only union static property value with a lambda, producing StaticLazyProperty["Demo"](lambda: Demo_A("prop")). This avoids the forward reference and matches F# getter semantics (re-evaluated per access). Scoped to getter-only to preserve settable-field semantics.
• transformStaticProperty: emit the descriptor type annotation as a string forward reference whenever the property type is the enclosing union (compare by FullName, not only DisplayName).
• makeCallWithArgInfo (FSharp2Fable.Util.fs): when referencing a union to reach a static member in Python, target the _Demo base class instead of the Demo type alias — the same underscore convention already used for reflection. This change is gated to Python + IsFSharpUnion and is a no-op for every other target.

Verification

• New regression test Issue4634 in tests/Python/TestNonRegression.fs.
• Full Python suite: 2351 passed.
• Pyright: no new errors introduced (the pre-existing 34 are in unrelated files).
• Generated module now imports and runs: x = A "prop", y = A "meth".

🤖 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

[dbrattli]

Follow-up: cross-module static member access was still broken

While adding a cross-module regression test, I found the original fix only handled the same-file case. entityIdent yields a bare IdentExpr for a same-file union but an Import for a union defined in another file — and the Import case fell through to the type alias, so static member access across modules still raised:

AttributeError: 'typing.TypeAliasType' object has no attribute 'prop_default'

Changes (commit 7733d0594)

• FSharp2Fable.Util.fs — added an Import arm alongside the IdentExpr arm that rewrites the import Selector to the underscore-prefixed base class (_Demo). transformImport uses the selector string as the imported name, so cross-file references now resolve to the base class that actually holds the static members.
• MiscTestsHelper.fs + TestNonRegression.fs — added a cross-module regression test: CrossModuleDemo is defined in a separately-compiled module and its static property/method are referenced from TestNonRegression.fs. This exercises the Import path; the prior commit's test only covered same-file references.
• Fable2Python.Transforms.fs — minor cleanup: replaced the dense inline let … in inside the when guard with a named isSelfReference binding.

Generated output (cross-module)

from misc_tests_helper import (CrossModuleDemo_A, _CrossModuleDemo)
...
Testing_equal(CrossModuleDemo_A("prop"), _CrossModuleDemo.prop_default)
Testing_equal(CrossModuleDemo_A("meth"), _CrossModuleDemo.meth_default())

Verification

• Full Python suite: 2352 passed (was 2351; +1 for the new cross-module test, now green).

Note

build.sh test python returned exit code 0 even while the new test was failing — the failure only showed in the pytest summary line, not the process exit. Worth keeping in mind for CI.

🤖 Generated with Claude Code