RuntimeError: dictionary changed size during iteration

dictionary-changed-size-during-iteration.py

from threading import Thread
from time import sleep

mydict = {}


def x():
    while True:
        # RuntimeError: dictionary changed size during iteration
        for k in mydict.keys():
            print(k)
            sleep(2)


def y():
    while True:
        for i in range(1000):
            mydict[i] = i
            sleep(1)


t1 = Thread(target=x)
t2 = Thread(target=y)
t1.start()
t2.start()
t2.join()
t1.join()