property() accepts a fifth positional argument instead of raising TypeError #8460

Feature

property(None, None, None, None, None) constructs a property object in RustPython, but CPython raises TypeError because property() takes at most four arguments.

Reproduction

print(type(property(None, None, None, None, None)))

RustPython:

<class 'property'>

CPython:

TypeError: property() takes at most 4 arguments (5 given)

Root cause

PropertyArgs declares a fifth #[pyarg(any, default)] field, name, and compute_arity_bounds counts every positional-or-keyword field toward the maximum arity, so PropertyArgs::arity() is 0..=5 and the leftover-positional check in FuncArgs::bind passes for five arguments; Initializer::init then stores the fifth one in the __name__ slot. CPython's property.__init__ is Argument Clinic generated with maxpos = 4 and the keyword list {fget, fset, fdel, doc}, so _PyArg_UnpackKeywords rejects the fifth argument before property_init_impl runs. The same field also makes RustPython accept property(name='x'), which is not in CPython's keyword list either.

Fix

Remove the name field from PropertyArgs and the assignment that consumes it in Initializer::init, which drops the derived arity back to 0..=4. No caller passes it: clone_property_with already passes name: None and copies the name separately, and the PyProperty name slot stays filled by __set_name__ and the __name__ setter.

Environment

• RustPython d248a04 (Python 3.14.0), also reproduced on 0.5.0
• CPython v3.14.3
• OS: Debian 12

Python Documentation or reference to CPython source code

• CPython property_init: Objects/clinic/descrobject.c.h#L133-L150 — the Argument Clinic generated parser fixes the keyword list at four entries and passes maxpos = 4:

  static const char * const _keywords[] = {"fget", "fset", "fdel", "doc", NULL};
  static _PyArg_Parser _parser = {
      .keywords = _keywords,
      .fname = "property",
      .kwtuple = KWTUPLE,
  };
  ...
  fastargs = _PyArg_UnpackKeywords(_PyTuple_CAST(args)->ob_item, nargs, kwargs, NULL, &_parser,
          /*minpos*/ 0, /*maxpos*/ 4, /*minkw*/ 0, /*varpos*/ 0, argsbuf);

• CPython _PyArg_UnpackKeywords: Python/getargs.c#L2429-L2441 — raises the TypeError once the argument count exceeds maxargs, before the implementation function is reached:

  if (!varpos && nargs + nkwargs > maxargs) {
      /* Adding "keyword" (when nargs == 0) prevents producing wrong error
         messages in some special cases (see bpo-31229). */
      PyErr_Format(PyExc_TypeError,
                   "%.200s%s takes at most %d %sargument%s (%zd given)",
                   (parser->fname == NULL) ? "function" : parser->fname,
                   (parser->fname == NULL) ? "" : "()",
                   maxargs,
                   (nargs == 0) ? "keyword " : "",
                   (maxargs == 1) ? "" : "s",
                   nargs + nkwargs);
      return NULL;
  }

• Documentation: property, whose signature is property(fget=None, fset=None, fdel=None, doc=None).

Related arity-validation gaps in other builtin constructors: #8461, #8462.

AI disclosure: this report was drafted with Claude Code (claude-opus-5) and reviewed by me before submission. The divergence comes from a differential-testing run against CPython 3.14.3, and every source reference above was checked against the linked revisions.