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Python多线程使用和注意事项

2019-01-21 02:44:54来源：博客园阅读 ()

多线程

基本实现：

第一种，函数方式

# -*- coding:utf-8 -*-
import thread
import time
 
 
def print_time(threadName, delay):
    count = 0
    while count < 5:
        time.sleep(delay)
        count += 1
        print '%s : %s' % (threadName, time.ctime(time.time()))
 
 
try:
    thread.start_new_thread(print_time, ("Thread-1", 2,))
    thread.start_new_thread(print_time, ("Thread-2", 4,))
except:
    print "Error!Unable to start thread."
 
while 1:
    pass

第二种，继承父类

# -*- coding:utf-8 -*-
import threading
import time
 
 
class MyThread(threading.Thread):
    def __init__(self, thread_id, name, counter):
        threading.Thread.__init__(self)
        self.thread_id = thread_id
        self.name = name
        self.counter = counter
 
    def run(self):
        print "Starting:" + self.name
        print_time(self.name, self.counter, 5)
        print "Exiting:" + self.name
 
 
def print_time(thread_name, delay, counter):
    while counter:
        time.sleep(delay)
        print '%s : %s' % (thread_name, time.ctime(time.time()))
        counter -= 1
 
 
thread1 = MyThread(1, "Thread-1", 1)
thread2 = MyThread(2, "Thread-2", 2)
 
thread1.start()
thread2.start()

线程同步的问题解决：锁

这里第一个线程执行的时候，第二个线程是等待状态的

# -*- coding:utf-8 -*-
import threading
import time
 
threadLock = threading.Lock()
threads = []
 
 
class MyThread(threading.Thread):
    def __init__(self, thread_id, name, counter):
        threading.Thread.__init__(self)
        self.thread_id = thread_id
        self.name = name
        self.counter = counter
 
    def run(self):
        print "Starting:" + self.name
        threadLock.acquire()
        print_time(self.name, self.counter, 5)
        print "Exiting:" + self.name
        threadLock.release()
 
 
def print_time(thread_name, delay, counter):
    while counter:
        time.sleep(delay)
        print '%s : %s' % (thread_name, time.ctime(time.time()))
        counter -= 1
 
 
thread1 = MyThread(1, "Thread-1", 1)
thread2 = MyThread(2, "Thread2", 2)
 
thread1.start()
thread2.start()
 
threads.append(thread1)
threads.append(thread2)
 
for thread in threads:
    thread.join()

线程优先级队列：

虽然开启了多个线程，不过打印顺序一定是：one按顺序到five

# -*- coding:utf-8 -*-
import threading
import time
import Queue
 
exit_flag = 0
queue_lock = threading.Lock()
work_queue = Queue.Queue(10)
thread_list = ["Thread-1", "Thread-2", "Thread-3"]
name_list = ["one", "two", "three", "four", "five"]
threads = []
thread_id = 1
 
 
class MyThread(threading.Thread):
    def __init__(self, thread__id, name, queue):
        threading.Thread.__init__(self)
        self.thread__id = thread__id
        self.name = name
        self.queue = queue
 
    def run(self):
        print "Starting:" + self.name
        process_data(self.name, self.queue)
        print "Exiting:" + self.name
 
 
def process_data(thread_name, queue):
    while not exit_flag:
        queue_lock.acquire()
        if not work_queue.empty():
            data = queue.get()
            queue_lock.release()
            print "%s processing %s" % (thread_name, data)
        else:
            queue_lock.release()
        time.sleep(2)
 
 
for t in thread_list:
    thread = MyThread(thread_id, t, work_queue)
    thread.start()
    threads.append(thread)
    thread_id += 1
 
queue_lock.acquire()
for word in name_list:
    work_queue.put(word)
queue_lock.release()
 
while not work_queue.empty():
    pass
 
exit_flag = 1
 
for t in threads:
    t.join()

这里的join函数重点解释下：

join的原理就是依次检验线程池中的线程是否结束，没有结束就阻塞主线程直到其他线程结束，如果结束则跳转执行下一个线程的join函数

接下来看看多线程实际的案例：

多线程访问网站

# -*- coding:utf-8 -*-
import urllib2
import time
from threading import Thread
 
 
class GetUrlThread(Thread):
    def __init__(self, url):
        Thread.__init__(self)
        self.url = url
 
    def run(self):
        response = urllib2.urlopen(self.url)
        print self.url, response.getcode()
 
 
def get_responses():
    urls = [
        'https://www.baidu.com',
        'https://www.taobao.com',
        'https://www.cnblogs.com',
        'https://github.com',
        'https://www.jd.com'
    ]
    start = time.time()
    threads = []
    for url in urls:
        thread = GetUrlThread(url)
        threads.append(thread)
        thread.start()
 
    for thread in threads:
        thread.join()
 
    print "Time: % s" % (time.time() - start)
 
 
get_responses()

如果多个线程访问同一个变量，容易出问题，比如下面：

有可能最后的实际值并不是50

# -*- coding:utf-8 -*-
from threading import Thread
 
some_var = 0
 
 
class IncrementThread(Thread):
    def run(self):
        global some_var
        read_value = some_var
        print "线程%s中的some_var是%d" % (self.name, read_value)
        some_var = read_value + 1
        print "线程%s中的some_var增加后变成%d" % (self.name, some_var)
 
 
def use_increment_thread():
    threads = []
    for i in range(50):
        thread = IncrementThread()
        threads.append(thread)
        thread.start()
 
    for thread in threads:
        thread.join()
 
    print "在50次运算后some_var应该变成50"
    print "在50次运算后some_var实际值为:%d" % (some_var,)
 
 
use_increment_thread()

解决办法，加入一个锁：

这种情况，最后的实际值一定是50

# -*- coding:utf-8 -*-
from threading import Thread, Lock
 
lock = Lock()
some_var = 0
 
 
class IncrementThread(Thread):
    def run(self):
        global some_var
        lock.acquire()
        read_value = some_var
        print "线程%s中的some_var是%d" % (self.name, read_value)
        some_var = read_value + 1
        print "线程%s中的some_var增加后变成%d" % (self.name, some_var)
        lock.release()
 
 
def use_increment_thread():
    threads = []
    for i in range(50):
        thread = IncrementThread()
        threads.append(thread)
        thread.start()
 
    for thread in threads:
        thread.join()
 
    print "在50次运算后some_var应该变成50"
    print "在50次运算后some_var实际值为:%d" % (some_var,)
 
 
use_increment_thread()

另一个锁的案例：

不加锁容易出事

# -*- coding:utf-8 -*-
from threading import Thread
import time
 
 
class CreateListThread(Thread):
    def __init__(self):
        self.entries = []
        Thread.__init__(self)
 
    def run(self):
        self.entries = []
        for i in range(10):
            time.sleep(1)
            self.entries.append(i)
        print self.entries
 
 
def use_create_list_thread():
    for i in range(3):
        t = CreateListThread()
        t.start()
 
 
use_create_list_thread()

结果：

[[[000, , , 111, , , 222, , , 333, , , 444, , , 555, , , 666, , , 777, , , 888, , , 999]]]

给他加上锁：

# -*- coding:utf-8 -*-
from threading import Thread, Lock
import time
 
lock = Lock()
 
 
class CreateListThread(Thread):
    def __init__(self):
        self.entries = []
        Thread.__init__(self)
 
    def run(self):
        self.entries = []
        for i in range(10):
            time.sleep(1)
            self.entries.append(i)
        lock.acquire()
        print self.entries
        lock.release()
 
 
def use_create_list_thread():
    for i in range(3):
        t = CreateListThread()
        t.start()
 
 
use_create_list_thread()