import unittest
from collections import deque
from copy import copy
from test.support import threading_helper

threading_helper.requires_working_threading(module=True)


class TestDeque(unittest.TestCase):
    def test_copy_race(self):
        # gh-144809: Test that deque copy is thread safe. It previously
        # could raise a "deque mutated during iteration" error.
        d = deque(range(100))

        def mutate():
            for i in range(1000):
                d.append(i)
                if len(d) > 200:
                    d.popleft()

        def copy_loop():
            for _ in range(1000):
                copy(d)

        threading_helper.run_concurrently([mutate, copy_loop])


if __name__ == "__main__":
    unittest.main()