在Python下尝试多线程编程

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我们前面提到了进程是由若干线程组成的，一个进程至少有一个线程。

由于线程是操作系统直接支持的执行单元，因此，高级语言通常都内置多线程的支持，Python也不例外，并且，Python的线程是真正的Posix Thread，而不是模拟出来的线程。

Python的标准库提供了两个模块：thread和threading，thread是低级模块，threading是高级模块，对thread进行了封装。绝大多数情况下，我们只需要使用threading这个高级模块。

启动一个线程就是把一个函数传入并创建Thread实例，然后调用start()开始执行：

import time, threading

# 新线程执行的代码:
def loop():
  print 'thread %s is running...' % threading.current_thread().name
  n = 0
  while n < 5:
    n = n + 1
    print 'thread %s >>> %s' % (threading.current_thread().name, n)
    time.sleep(1)
  print 'thread %s ended.' % threading.current_thread().name

print 'thread %s is running...' % threading.current_thread().name
t = threading.Thread(target=loop, name='LoopThread')
t.start()
t.join()
print 'thread %s ended.' % threading.current_thread().name

thread MainThread is running...
thread LoopThread is running...
thread LoopThread >>> 1
thread LoopThread >>> 2
thread LoopThread >>> 3
thread LoopThread >>> 4
thread LoopThread >>> 5
thread LoopThread ended.
thread MainThread ended.

import time, threading

 
# 假定这是你的银行存款:
balance = 0

def change_it(n):
  # 先存后取，结果应该为0:
  global balance
  balance = balance + n
  balance = balance - n

def run_thread(n):
  for i in range(100000):
    change_it(n)

t1 = threading.Thread(target=run_thread, args=(5,))
t2 = threading.Thread(target=run_thread, args=(8,))
t1.start()
t2.start()
t1.join()
t2.join()
print balance

balance = balance + n

x = balance + n
balance = x

t1: x1 = balance + 5 # x1 = 0 + 5 = 5
t1: balance = x1   # balance = 5
t1: x1 = balance - 5 # x1 = 5 - 5 = 0
t1: balance = x1   # balance = 0

t2: x2 = balance + 8 # x2 = 0 + 8 = 8
t2: balance = x2   # balance = 8
t2: x2 = balance - 8 # x2 = 8 - 8 = 0
t2: balance = x2   # balance = 0

t1: x1 = balance + 5 # x1 = 0 + 5 = 5

t2: x2 = balance + 8 # x2 = 0 + 8 = 8
t2: balance = x2   # balance = 8

t1: balance = x1   # balance = 5
t1: x1 = balance - 5 # x1 = 5 - 5 = 0
t1: balance = x1   # balance = 0

t2: x2 = balance - 5 # x2 = 0 - 5 = -5
t2: balance = x2   # balance = -5

balance = 0
lock = threading.Lock()

def run_thread(n):
  for i in range(100000):
    # 先要获取锁:
    lock.acquire()
    try:
      # 放心地改吧:
      change_it(n)
    finally:
      # 改完了一定要释放锁:
      lock.release()

import threading, multiprocessing

def loop():
  x = 0
  while True:
    x = x ^ 1

for i in range(multiprocessing.cpu_count()):
  t = threading.Thread(target=loop)
  t.start()

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