map(f) with a single argument builds a map object instead of raising TypeError #8462

Feature

map() called with one positional argument returns a map object in RustPython, but CPython raises TypeError.

Reproduction

print(type(map({})))

RustPython:

<class 'map'>

CPython:

TypeError: map() must have at least two arguments.

Root cause

PyMap::py_new collects the trailing positional arguments into PosArgs<PyIter> and stores the resulting vector without checking that at least one iterable was supplied, so map(f) constructs a map object out of a single argument, and PyMap::next does not special-case an empty iterator vector either, so that object is an iterator calling f() with no arguments forever. CPython validates the count before touching any argument, in both map_vectorcall and map_new, and converts arguments 2..n with PyObject_GetIter only after that check passes. The neighbouring iterators do bound the count: zip.rs special-cases the empty vector, and filter.rs makes the iterable mandatory at the type level with type Args = (PyObjectRef, PyIter).

Fix

In PyMap::py_new, return vm.new_type_error("map() must have at least two arguments.") when the collected iterator vector is empty, rather than checking in IterNext::next, since CPython reports this as an argument error at call time.

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 map_vectorcall: Python/bltinmodule.c#L1423-L1434 — rejects fewer than two positional arguments before any argument is converted to an iterator:

  Py_ssize_t nargs = PyVectorcall_NARGS(nargsf);
  if (kwnames != NULL && PyTuple_GET_SIZE(kwnames) != 0) {
      // Fallback to map_new()
      PyThreadState *tstate = _PyThreadState_GET();
      return _PyObject_MakeTpCall(tstate, type, args, nargs, kwnames);
  }
  
  if (nargs < 2) {
      PyErr_SetString(PyExc_TypeError,
         "map() must have at least two arguments.");
      return NULL;
  }

• CPython map_new: Python/bltinmodule.c#L1382-L1387 — repeats the same check on the keyword-argument path that map_vectorcall falls back to, so both entry points reject the call.

• Documentation: builtins.map

Related arity-validation gaps in other builtin constructors: #8460, #8461.

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.