Several built-in exceptions store their attributes in the instance dict instead of as fields #8345

Feature

Several built-in exceptions store their attributes in the instance __dict__ instead of as struct fields, so Exc().__dict__ is non-empty (unlike CPython) and the attributes are not backed by descriptors. SystemExit.code (#8230 / #8282) and StopIteration.value (#8290 / #8301) have been converted already; this issue tracks the rest.

Measured against CPython 3.14.6 (print(exc.__dict__)):

Their subclasses inherit the same problem (ModuleNotFoundError from ImportError; IndentationError / TabError / _IncompleteInputError from SyntaxError) and should be fixed automatically once the base is converted.

These are the remaining cases of the issue addressed in #8282 and #8301.
SyntaxError already carries a // TODO: members note for this.

Plan

Convert each to a struct field (#[repr(C)] + base field, exposed via #[pygetset]), following the approach established in #8282 and #8301, in separate PRs:

1. NameError and AttributeError
   • Both are small and use a similar keyword-argument pattern.
2. ImportError
   • ModuleNotFoundError will inherit the converted fields.
3. UnicodeDecodeError, UnicodeEncodeError, and UnicodeTranslateError
   • These exceptions share most of their fields, similar to CPython's implementation.
4. SyntaxError
   • This will also cover IndentationError, TabError, and _IncompleteInputError.
   • It is the largest conversion and will require checking the code paths that read these attributes.

Python Documentation or reference to CPython source code

https://docs.python.org/3/library/exceptions.html

These attributes are defined via PyMemberDef (struct members, not instance-dict entries) in CPython's Objects/exceptions.c.

Verified locally against CPython 3.14.6 and RustPython. AI was used to help organize the initial draft; the technical details and final wording were reviewed by me.