Feature

As itertools.tee generates n independent iterators from a single iterable object, each IterNext of itertools.tee object will access the shallow-copy of PyItertoolsTeeData .
We need to prevent re-entering get_item method of PyItertoolsTeeData during iteration for keeping PyItertoolsTeeData thread-safety.

CPython use running variable whether the current entering is re-entering or first-entering like below code

// In a middle of get_item method of PyItertoolsTeeData
if (tdo->running) {
    PyErr_SetString(PyExc_RuntimeError,
                    "cannot re-enter the tee iterator");
    return NULL;
}
tdo->running = 1;
value = PyIter_Next(tdo->it);
tdo->running = 0;

Python Documentation

• https://bugs.python.org/issue34410
• bpo-34410: Fix a crash in the tee iterator when re-enter it. python/cpython#15625
• https://github.com/python/cpython/blob/3.10/Modules/itertoolsmodule.c#L577-L646

Relevant Test Case

• test_itertools.py test_tee_reenter