from weakref import WeakSet
from threading import Thread

from random import random, randrange, shuffle
from time import sleep
from itertools import count


def f1(ws):
    list(ws)


def f2(ws):
    sleep(random())
    list(ws)


def g(s):
    sleep(random())
    s.clear()


class Obj:
    pass


for c in count(1):
    # the more things we add to the set
    # the more work _commit_removals is doing
    # so the larger the window for another iteration to start
    s = set(Obj() for _ in range(randrange(100_000, 1_000_000)))
    ws = WeakSet(s)

    # we make 2 threads that try iterate the weakset at the same time
    # (actually we want there to be a slight offset so that the second starts just after the first finishes)
    # more `f` threads won't necessarily help observe it, as we require a moment where *no* other threads are iterating to hit the race
    ts = [
        Thread(target=f1, args=[ws]),
        Thread(target=f2, args=[ws]),
        Thread(target=g, args=[s]),
    ]

    shuffle(ts)

    for t in ts:
        t.start()

    for t in ts:
        t.join()

    if c % 80 == 0:
        print()
    print(".", end="", flush=True)

EX_OUTPUT = """\
...............................................................................
................................................................................
.......Exception in thread Thread-500:
Traceback (most recent call last):
  File "/home/vscode/.pyenv/versions/3.9.6/lib/python3.9/threading.py", line 973, in _bootstrap_inner
    self.run()
  File "/home/vscode/.pyenv/versions/3.9.6/lib/python3.9/threading.py", line 910, in run
    self._target(*self._args, **self._kwargs)
  File "/home/vscode/threading_weakset_concurrent_iter.py", line 15, in f2
    list(ws)
  File "/home/vscode/.pyenv/versions/3.9.6/lib/python3.9/_weakrefset.py", line 61, in __iter__
    for itemref in self.data:
RuntimeError: Set changed size during iteration
"""