Summary

inspect.signature() fails on common builtins (len, sorted, open, …) with ValueError: <...> builtin has invalid signature, where CPython returns a proper Signature. The cause is that the auto-generated __text_signature__ values are malformed: they expose the Rust parameter names and omit the positional-only (/) and keyword-only (*) markers, so they are either rejected by inspect or semantically wrong.

Reproduction

import inspect
print(inspect.signature(len))
# RustPython: ValueError: <built-in function len> builtin has invalid signature
# CPython:    (obj, /)

Same ValueError for inspect.signature(sorted) and inspect.signature(open).

The underlying __text_signature__ strings:

RustPython actually populates __text_signature__ for more builtins than CPython, but the values are generated from the Rust function's parameters (opts, args, iter_target, …) and lack the / / * separators, so:

• inspect.signature raises ValueError instead of returning a signature, and
• where it would parse, the signature is wrong (e.g. sorted would show (iterable, opts) rather than (iterable, /, *, key=None, reverse=False)).

Context

This is a follow-up to #2410 ("Builtin functions and methods lack a .__text_signature__ property"), which is closed — the attribute now exists, but its values do not match CPython and break inspect.signature.

Producing fully accurate signatures for every builtin is a large effort (CPython hand-authors them via Argument Clinic), so a reasonable minimum would be that inspect.signature() returns something usable (or raises the same way CPython does for genuinely signature-less builtins) rather than ValueError on len/sorted/open.

Reference

Verified against CPython 3.14.5.

Investigated and drafted by Claude; reviewed before filing.