fix(py_class): support super().__init__() in init=False subclasses

[junrushao]

Summary

• Fix segfault when @py_class(init=False) subclasses use super().__init__() followed by field assignment
• Add ffi.NewEmpty C++ function to allocate zero-initialized objects by type index
• Detect super-init-from-subclass pattern in _make_init() and pre-allocate instead of dispatching to __ffi_init__

Motivation

The common Python pattern of defining a custom __init__ that calls super().__init__() then sets fields crashes when used with @py_class(init=False):

@py_class(init=False)
class PointerType(Node):
    element_type: Object

    def __init__(self, element_type):
        super().__init__()       # parent's auto-init dispatches to child's C++ ctor → crash
        self.element_type = element_type

The parent's auto-generated __init__ forwards to __ffi_init__ using the child type's C++ constructor, which expects field arguments that were not provided.

Design

C++ side (src/ffi/extra/dataclass.cc): Register ffi.NewEmpty(type_index) -> ObjectRef that allocates a zero-initialized object via CreateEmptyObject. The calloc'd state is valid: None for Any/ObjectRef fields, 0 for scalars.

Python side (python/tvm_ffi/registry.py):

• _ffi_alloc_empty(obj): Calls ffi.NewEmpty to allocate an empty FFI object for type(obj) and moves the handle into obj. No-op if already allocated.
• _is_super_init_from_subclass(self): Compares type(self).__tvm_ffi_type_info__ identity against the declaring class's type_info. Returns True only for registered @py_class subclasses — undecorated subclasses inherit the parent's type_info so the identity check correctly filters them out.
• Both _make_init code paths (with/without __post_init__) intercept the no-args super-init-from-subclass call and allocate an empty object instead of dispatching to __ffi_init__.

Test plan

• uv run pytest -vvs tests/python/test_dataclass_py_class.py::TestSuperInitPattern — 12 tests pass
• uv run pytest -vvs tests/python/ — 1970 passed, 38 skipped, 3 xfailed, 0 failures

Tests cover: basic super-init, deep hierarchy (Node→BaseType→PointerType), calloc defaults, intermediate custom/auto inits, normal init unaffected, non-py_class subclass error handling, isinstance checks, field overwrite, copy/deepcopy, and direct-from-Object inheritance.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request enables support for the super().__init__() pattern in @py_class(init=False) subclasses by introducing a mechanism to pre-allocate empty, zero-initialized FFI objects. This prevents crashes that previously occurred when calling parent constructors without field arguments. The implementation includes a new C++ helper ffi.NewEmpty and updates to the Python registry to handle object allocation during initialization. I have provided a suggestion to use functools.wraps in the _install_init wrapper to better preserve metadata during function wrapping.