Prove that a class is not thread-safe.

test.py

#!/usr/bin/env python2.7
# -*- coding: utf-8 -*-


import threading
import Queue
import time


class MySingletonNTSafe(object):
    """implement Singleton pattern, not-thread-safe"""
    _instance = None

    def __new__(cls, *args, **kwargs):
        print('Time to sleep: %f' % (kwargs['stime'],))
        if MySingletonNTSafe._instance is None:
            time.sleep(kwargs['stime'])
            MySingletonNTSafe._instance = super(MySingletonNTSafe, cls).__new__(cls)

        return MySingletonNTSafe._instance

    def __init__(self, *args,**kwargs):
        MySingletonNTSafe._instance = self


class Test(object):
    """Test whether the class MySingleton works"""
    def __init__(self, tnum=3):
        self.tnum = tnum
        self.queue_st = Queue.Queue()
        self.queue_nst = Queue.Queue()

    def run(self):
        threads_ns = []
        threads_ns.append(threading.Thread(target=self._run_nst, kwargs={'stime': 2}))
        threads_ns.append(threading.Thread(target=self._run_nst, kwargs={'stime': 0}))
        for t in threads_ns:
            t.start()

        for t in threads_ns:
            t.join()

    def _run_nst(self, stime=0):
        # not-thread-safe case
        obj = MySingletonNTSafe(stime=stime)
        self.queue_nst.put(obj)


if __name__ == '__main__':
    num_try = 0
    flag = False
    while True:
        test = Test()
        test.run()
        
        objs_nst = []
        while not test.queue_nst.empty():
            objs_nst.append(test.queue_nst.get())
        last = objs_nst.pop()
        for obj in objs_nst:
            if not last == obj:
                print('NOT IDENTICAL')
                flag = True
                break

        num_try += 1
        print('Number of try: #%d' % (num_try,))
                    
        if flag:
            break

        print('\n')
        time.sleep(0.5)