• Closes SystemExit.code is read-only; CPython allows assignment #8230
• This PR follows our AI policy

Summary

SystemExit.code was a read-only getset recomputed from args on
every access, so assigning or deleting exc.code raised
AttributeError, and mutating exc.args after init changed code.
It is now stored in a dedicated struct field (#[repr(C)] with a
PyBaseException base, the same approach as OSError), set at
__init__ time — writable, deletable, and independent from args
after init, as in CPython. Keeping it in a field rather than the
instance dict leaves SystemExit().__dict__ empty, matching CPython.

A second commit updates handle_exit_exception to read the exit
status from the code attribute like CPython's handle_system_exit;
without it, reassigning exc.code would not affect the process exit
status:

import sys
try:
    sys.exit(1)
except SystemExit as e:
    e.code = 0   # before: AttributeError: ... not writable
    raise
# process exit status — before: 1, after: 0 (matches CPython)

Test plan

• Verified against CPython 3.14.6: attribute write/delete including the subclass repro from SystemExit.code is read-only; CPython allows assignment #8230, exit statuses for sys.exit() /
  sys.exit(3) / sys.exit('msg') / raise SystemExit(1, 2), and the re-raise case above.
• Lib/test/test_exceptions.py, Lib/test/test_sys.py: no regressions (the single test_badisinstance failure is pre-existing on main).
• test_threading / test_io thread-finalization cases pass (SystemExit is
  now created via the full constructor path, not new_exception).
• cargo fmt / cargo clippy: clean (no new warnings).

Assisted-by: Claude Code:claude-fable-5

Summary by CodeRabbit

Summary by CodeRabbit

• Bug Fixes
  • Improved handling of SystemExit during termination, including finalization and non-main threads.
  • Exit codes are now derived from SystemExit.code (with None mapping to 0; integer values supported).
  • SystemExit output is generated more reliably from the code value when present.
  • SystemExit created via exit (and internal thread exit) now follows the same raising path, ensuring consistent behavior with provided exit values.