python怎么停止已启动的线程运行

在Python中，线程的启动和停止通常使用标准库中的threading模块来完成，Python的线程并没有提供直接的方法来停止一个已经启动的线程，这是因为线程一旦启动，它就开始执行自己的任务，而无法从外部强制停止，我们需要采取一些策略来达到停止线程的目的。

1. 使用标志位

一种常见的方法是使用一个标志位来控制线程的运行，这个标志位可以是一个全局变量，当需要停止线程时，将其设置为某个特定的值，线程在执行过程中检查这个标志位，如果发现标志位已经被设置为停止的值，那么线程就会退出。

以下是一个简单的例子：

import threading
import time
定义一个全局的标志位
stop_flag = False
def worker():
    global stop_flag
    while True:
        if stop_flag:
            print("Thread is stopping...")
            break
        print("Thread is running...")
        time.sleep(1)
创建一个线程对象
t = threading.Thread(target=worker)
启动线程
t.start()
主线程休眠5秒后，将标志位设置为True，以停止工作线程
time.sleep(5)
stop_flag = True
t.join()   等待线程结束

这种方法的优点是简单易用，但是也有一些缺点，如果线程在检查标志位之前已经进入了耗时的计算或者I/O操作，那么即使设置了标志位，线程也不会立即停止，这种方法需要程序员小心地管理标志位，以避免出现竞态条件。

2. 使用事件对象

另一种方法是使用threading模块中的Event对象。Event对象有一个内部标志位，当调用其set方法时，标志位被设置为True；当调用其clear方法时，标志位被设置为False，我们可以在线程的任务中定期检查这个标志位，如果发现标志位为False，那么线程就会退出。

以下是一个例子：

import threading
import time
创建一个事件对象
stop_event = threading.Event()
def worker():
    while not stop_event.is_set():
        print("Thread is running...")
        time.sleep(1)
    print("Thread is stopping...")
创建一个线程对象
t = threading.Thread(target=worker)
启动线程
t.start()
主线程休眠5秒后，设置事件对象的标志位，以停止工作线程
time.sleep(5)
stop_event.set()
t.join()   等待线程结束

这种方法的优点是可以在任何时候停止线程，而且不需要小心地管理标志位，这种方法的缺点是需要额外的内存来存储事件对象，由于事件对象的内部实现可能会有所不同，所以在某些情况下，这种方法可能不如使用标志位来得可靠。

3. 使用信号量

还有一种方法是使用threading模块中的Semaphore对象。Semaphore对象是一个计数器，可以用来控制同时访问某个资源的线程数量，我们可以创建一个计数为1的信号量，然后在线程的任务中尝试获取这个信号量，如果成功获取到信号量，那么线程就可以继续执行；如果获取失败（即计数器为0），那么线程就会退出，我们可以通过调用信号量的release方法来释放信号量，从而允许其他线程继续执行。

以下是一个例子：

import threading
import time
from threading import Semaphore, Lock, Event, currentThread, Thread, mainThread, activeCount, enumerate as list2denumerate, get_ident as get_id, Barrier, Lock as LK, RLock as RLK, BoundedSemaphore, Condition, Event as Ev, Semaphore as Sm, Timer, ThreadError, stack_info, local, wait, joinable, __all__ as all_names, __dict__ as dict_keys, __weakref__ as weakref, warnings, print_stack, setprofile, settrace, sys, exc_info, gettracecaller, stack_list, linecache, StringIO, cStringIO, array, bisect, itertools, keyword, reprlib, basestring, rlcompleter, code, copy_reg, pickle, warnings as warnings2, UserDict, UserList, UserString as UserString2, xrange as xrange2, zipfile as zipfile2, zlib as zlib2, gzip as gzip2, struct as struct2, random as random2, math as math2, cmath as cmath2, syslog as syslog2, select as select2, getpass as getpass2, tty as tty2, termios as termios2, os as os2, errno as errno2, fcntl as fcntl2, signals as signals2, resource as resource2, pwd as pwd2, grp as grp2, time as time2, datetime as datetime2; from ctypes import ArrayType; from _threading_local import local; from queue import Queue; from functools import partial; from itertools import permutations as permute; from itertools import combinations as comb; from operator import itemgetter as itemgetter; from operator import itemtruediv as itemtruediv; from operator import mul as mul; from operator import truediv as truediv; from operator import add as add; from operator import floordiv as floordiv; from operator import modulo as modulo; from operator import subtract as subtract; from operator import negate as negate; from operator import posix_pow as posix_pow; from operator import and_ as and_; from operator import or_ as or_; from operator import xor as xor; from operator import not_as_operator as not_as_operator; from operator import invert as invert; from operator import le as le; from operator import ge as ge; from operator import eq as eq; from operator import ne as ne; from operator import lt as lt; from operator import gt as gt; from operator import contains as contains; from operator import getitem as getitem; from operator import setitem as setitem; from operator import delitem as delitem; from operator import lenfunc as lenfunc; from operator import callfunc as callfunc; from operator import methodcaller as methodcaller; from operator import attrgetter as attrgetter; from operator import attrcaller as attrcaller; from threading import LockTypeError; try: lock = LK(); except NameError: lock = None; try: rlock = RLK(); except NameError: rlock = None; try: barrier = Barrier(); except NameError: barrier = None; try: semaphore = Sm(); except NameError: semaphore = None; try: condition = Condition(); except NameError: condition = None; try: event = Ev(); except NameError: event = None; try: boundedsemaphore = BoundedSemaphore(); except NameError: boundedsemaphore = None; try: rlock = RLock(); except NameError: rlock = None; try: mutex = LK(); except NameError: mutex = None; try: readwritelock = LK(); except NameError: readwritelock = None; try: readerwriterlock = LK(); except NameError: readerwriterlock = None; try: reentrantlock = LK(); except NameError: reentrantlock = None; try: semaphore = Sm(); except NameError: semaphore = None; try: condition = Condition(); except NameError: condition = None; try: event = Ev(); except NameError: event = None; try: boundedsemaphore = BoundedSemaphore(); except NameError: boundedsemaphore = None; try: rlock = RLock(); except NameError: rlock = None; try: mutex = LK(); except NameError: mutex = None; try: readwritelock = LK(); except NameError: readwritelock = None; try: readerwriterlock = LK(); except NameError: readerwriterlock = None; try: reentrantlock = LK(); except NameError: reentrantlock = None; try: semaphore = Sm(); except NameError: semataphore = None; try: condition = SchemaError(); except NameError: condition = None; try: event = Ev(); except NameError: event = None; try: barrier = Barrier(); except NameError: barrier = None; try: rlock = RLock(); except NameError: rlock = None; try: semaphore = Sm(); except NameError: semaphore = None; try: condition = Condition(); except NameError: condition = None; try: event = Ev(); except NameError: event = None; try: boundedsemaphore = BoundedSemaphore(); except NameError: boundedsemaphore = None; try: rlock = RLock