René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"StopIteration.value is stored in the instance dict instead of as a member","articleBody":"## Feature\n\n`StopIteration.value` is stored in the instance `__dict__` instead of as a member (struct field) like CPython. This makes `StopIteration().__dict__` non-empty and causes `value` to return the wrong result when the dict is cleared.\n\nSame class of issue as `SystemExit.code` (#8230, fixed in #8282). Other built-in exceptions share it (`AttributeError`, `ImportError`, `SyntaxError`, `UnicodeDecodeError`); this issue targets `StopIteration` first.\n\n**Wrong value:**\n\n```python\ne = StopIteration(5)\ne.__dict__.clear()\nprint(e.value)\n# CPython 3.14: 5      (value is a member, unaffected by dict)\n# RustPython:   None   (value lives in the dict, so it's gone)\n```\n\n**Also observable:**\n\n```python\ntype(StopIteration.value)   # CPython: member_descriptor   RustPython: NoneType\nvars(StopIteration(5))      # CPython: {}                  RustPython: {'value': 5}\n```\n\n**Cause:** `value` is a class-level `None` set via `set_attr` into the instance dict in `slot_init`, instead of a struct field. The hot-path helper `VirtualMachine::new_stop_iteration` (`crates/vm/src/vm/vm_new.rs`) also allocates a fresh dict per call.\n\n**Proposed fix:** Move `value` to a struct field on `PyStopIteration` (the `#[repr(C)]` + base-field approach used by `PyOSError` and now `SystemExit` in #8282), exposed via `#[pygetset]`. Since `StopIteration` is raised on a very hot path, this also removes the per-instance dict allocation.\n\nI'll take this and open a PR, following the same approach as #8282.\n\n## Python Documentation or reference to CPython source code\n\nhttps://docs.python.org/3/library/exceptions.html#StopIteration\n\n`value` is defined as a `PyMemberDef` (a struct member, not an instance-dict entry) in CPython's `Objects/exceptions.c`.\n\n---\n*Reproduction verified against CPython 3.14.6 and RustPython locally. Issue drafted with AI assistance (Claude), reviewed and edited by me.*","author":{"url":"https://github.com/kangdora","@type":"Person","name":"kangdora"},"datePublished":"2026-07-16T03:17:00.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/8290/RustPython/issues/8290"}