Python wrapping import fails with system SWIG 4.4.1 under Limited API (moduledef/delete_SwigPyIterator); working window is SWIG 4.3.x

[hjmjohnson]

Description

Building ITK Python wrapping against system SWIG 4.4.1 (ITK_USE_SYSTEM_SWIG=ON) with the default Python Limited API / Stable ABI (ITK_USE_PYTHON_LIMITED_API=ON) compiles and links cleanly but fails at import with:

AttributeError: 'moduledef' object has no attribute 'delete_SwigPyIterator'

This is distinct from #6303 / #5540 (the SWIG 4.4.1 compile syntax error in ITKCommonBase.i), which is already fixed on main by 74902b3 (PR #5695, pygccxml + --castxml-output=1). That compile fix is confirmed present — the build reaches a full link — but the abi3 runtime glue from SWIG 4.4.1 references _pyBasePython.delete_SwigPyIterator, which the Limited-API moduledef module does not expose.

The verified system-SWIG compatibility window for ITK main (6.0.0) wrapping is SWIG 4.3.x only.

Verified build matrix (macOS arm64, ITK main 6.0.0, pixi-supplied swig/castxml, Limited API on)

System SWIG	Result	Cause
4.2.1	compile fails	error: use of undeclared identifier 'SWIG_Py_DECREF' in ITKCommonPython.cpp (macro absent pre-4.3 runtime)
2024-03-26-master (vendored)	works	the snapshot ITK CI is tuned to
4.3.1	works — imports + itk.median_image_filter runs	descends from the vendored master, has SWIG_Py_DECREF, pre-4.4 ABI codegen
4.4.1	import fails	4.4 Limited-API moduledef / delete_SwigPyIterator codegen change

CastXML was supplied from conda-forge 0.7.0 in all cases and worked.

Import traceback (SWIG 4.4.1)

File ".../itk/pyBasePython.py", line 88, in SwigPyIterator
    __swig_destroy__ = _pyBasePython.delete_SwigPyIterator
AttributeError: 'moduledef' object has no attribute 'delete_SwigPyIterator'

Generated glue header: # This file was automatically generated by SWIG ... Version 4.4.1.

Why CI does not catch this

ITK CI always uses the vendored 2024-03-26-master snapshot (Wrapping/Generators/SwigInterface/CMakeLists.txt:25), never a system SWIG ≥ 4.3. The Limited-API import path with system 4.4.1 is therefore untested upstream. #6303 was reported by a MacPorts build and only reached the compile stage.

Where a fix likely belongs (not igenerator.py)

The Limited-ABI codegen is driven by Wrapping/macro_files/itk_end_wrap_module.cmake (ITK_USE_PYTHON_LIMITED_API), Wrapping/Generators/Python/{main_module_ext,module_ext}.i.in, PyBase/pyBase.i, and CMake/ITKSetPython3Vars.cmake — not igenerator.py (whose 4.4.1-related change, PR #5695, was type-resolution, not ABI). SimpleITK builds with SWIG 4.4.1 and Limited API (SimpleITK_PYTHON_USE_LIMITED_API default ON) successfully, so its hand-written .i handling of the iterator protocol is a useful reference.

Suggested actions

• Raise swig_version_min from 4.2.0 (which cannot compile) to 4.3.0, and consider documenting 4.3.x as the supported system-SWIG window until 4.4 Limited-API glue is adapted.
• Track the 4.4.x Limited-API moduledef/SwigPyIterator adaptation as the remaining work to allow system SWIG ≥ 4.4.

Reproducibility

100% — clean ITK_USE_SYSTEM_SWIG=ON build with system SWIG 4.4.1, ITK_USE_PYTHON_LIMITED_API=ON, Python 3.13.

Versions

ITK main (6.0.0), Python 3.13, macOS arm64. conda-forge swig 4.4.1 / 4.3.1 / 4.2.1, castxml 0.7.0.

[hjmjohnson]

Fixed by #6484 (merged 2026-06-23), which moved the vendored SWIG to 4.4.1 and added a runtime shim for SWIG 4.4's PEP 489 multi-phase init (PyModule_FromDefAndSpec + PyModule_ExecDef in the injected module-packing code, Wrapping/Generators/Python/CMakeLists.txt). The shim is in ITK's generated glue, so it applies to system SWIG as well as the vendored binaries. System SWIG 4.4.1 + ITK_USE_PYTHON_LIMITED_API=ON now imports correctly on main.