Working fix

Author: rhettinger

Doc/library/itertools.rst

@@ -707,10 +707,6 @@ loops that truncate the stream.
             except StopIteration:
                 return
 
-   Once a :func:`tee` has been created, the original *iterable* should not be
-   used anywhere else; otherwise, the *iterable* could get advanced without
-   the tee objects being informed.
-
    When the input *iterable* is already a tee iterator object, all
    members of the return tuple are constructed as if they had been
    produced by the upstream :func:`tee` call.  This "flattening step"

Lib/test/test_itertools.py

@@ -1249,10 +1249,11 @@ def test_tee(self):
             self.assertEqual(len(result), n)
             self.assertEqual([list(x) for x in result], [list('abc')]*n)
 
-        # tee pass-through to copyable iterator
+        # tee objects are independent (see bug gh-123884)
         a, b = tee('abc')
         c, d = tee(a)
-        self.assertTrue(a is c)
+        e, f = tee(c)
+        self.assertTrue(len({a, b, c, d, e, f}) == 6)
 
         # test tee_new
         t1, t2 = tee('abc')

Modules/itertoolsmodule.c

@@ -1058,7 +1058,7 @@ itertools_tee_impl(PyObject *module, PyObject *iterable, Py_ssize_t n)
         Py_DECREF(result);
         return NULL;
     }
-    if (copyfunc != NULL) {
+    if (copyfunc != NULL && 0) {
         copyable = it;
     }
     else {