itertools.tee() calls iter() before checking n, and never calls it on the iterator it buffers #9512

Feature

itertools.tee() gets the order and the count of its iter() calls wrong: a negative n is reported only after iter(iterable) succeeds, and an iterator whose own __iter__ is broken is accepted. CPython checks n first, returns () for n == 0 without calling iter(), and then calls iter() once on the iterable and once more on the iterator it gets back.

Reproduction

import itertools


class NoneIter:
    def __iter__(self):
        return None


class BadIterator:
    def __iter__(self):
        return None

    def __next__(self):
        return None


class BadIterable:
    def __iter__(self):
        return BadIterator()


try:
    print(itertools.tee(NoneIter(), -1))
except Exception as e:
    print(f"{type(e).__name__}: {e}")
try:
    print(type(itertools.tee(BadIterable())))
except Exception as e:
    print(f"{type(e).__name__}: {e}")

RustPython:

TypeError: NoneIter.__iter__() must return an iterator, not NoneType
<class 'tuple'>

CPython:

ValueError: n cannot be negative
TypeError: BadIterator.__iter__() must return an iterator, not NoneType

tee(NoneIter(), 0) raises the same TypeError instead of returning (), and an iterator that defines __copy__ has it called by tee(), which CPython stopped doing in gh-123884.

Root cause

TeeArgs declares iterable as PyIter, so iter() runs during argument binding, before tee checks n. tee then copies an iterator with __copy__ directly, and PyItertoolsTee::from_iter stores any other iterator without calling iter() on it, where CPython's tee_fromiterable calls PyObject_GetIter on the iterator it is given.

Fix

Bind iterable as PyObjectRef, return an empty tuple for n == 0, call get_iter only after that, drop the __copy__ shortcut, and call get_iter again at the top of from_iter.

Environment

• RustPython 713e14c (Python 3.15.0a), also reproduced on 0.4.0 and 0.6.1
• CPython v3.15.2, also v3.14.4 and v3.12.13 (those say n must be >= 0 and iter() returned non-iterator of type 'NoneType')
• OS: macOS arm64 (713e14c), Debian 12 (0.4.0, 0.6.1)

Python Documentation or reference to CPython source code

• CPython itertools_tee_impl: Modules/itertoolsmodule.c#L1161-L1179 returns for n == 0 before calling iter(), after the generated wrapper has rejected a negative n (Modules/clinic/itertoolsmodule.c.h#L348-L352):

  result = PyTuple_New(n);
  if (result == NULL)
      return NULL;
  if (n == 0)
      return result;
  it = PyObject_GetIter(iterable);

• CPython tee_fromiterable: Modules/itertoolsmodule.c#L1051-L1058 calls iter() again on the iterator tee() passes it:

  it = PyObject_GetIter(iterable);
  if (it == NULL)
      return NULL;

• Documentation: itertools.tee

AI disclosure: this report was drafted with Claude Code (claude-opus-5-5) and reviewed by me before submission. The divergence comes from a differential-testing run against CPython 3.12.13, and every source reference above was checked against the linked revisions.