Question

目前的代码是：

def export_data(file):
    <runs the db2 database command to export tables to file>

def export_to_files(yaml):
    logger = logging.getLogger("export_to_files")
    thread1 = threading.Thread(target=export_data, args=[out_file1])
    thread1.start()
    thread2 = threading.Thread(target=export_data, args=[out_file2])
    thread2.start()
    thread1.join()
    thread2.join()

def main():
    export_to_files()

if __name__ == "__main__":
    main()

我的理解是join()只阻止调用线程。但是，我没有意识到thread1.join()甚至会阻止thread2执行，实质上使代码只运行1个线程，即thread1。

如何同时执行两个线程，同时主线程等待两者完成？

编辑：我纠正了，2个线程确实运行了，但似乎只有1个线程在某个时间点实际上“做”了事情。

进一步说明，callable_method正在从数据库中读取数据并写入文件。虽然我现在可以看到2个文件正在更新（每个线程写入一个单独的文件），但其中一个文件现在已经很长时间没有更新，而另一个文件是最新的当前时间。

正在使用否连接对象。查询从db2命令行界面运行。

Answer 1

您可以使用multiprocessing.pool中基本未记录的ThreadPool类来执行以下操作：

from multiprocessing.pool import ThreadPool
import random
import threading
import time

MAX_THREADS = 2
print_lock = threading.Lock()

def export_data(fileName):
    # simulate writing to file
    runtime = random.randint(1, 10)
    while runtime:
        with print_lock: # prevent overlapped printing
            print('[{:2d}] Writing to {}...'.format(runtime, fileName))
        time.sleep(1)
        runtime -= 1

def export_to_files(filenames):
    pool = ThreadPool(processes=MAX_THREADS)
    pool.map_async(export_data, filenames)
    pool.close()
    pool.join()  # block until all threads exit

def main():
    export_to_files(['out_file1', 'out_file2', 'out_file3'])

if __name__ == "__main__":
    main()

示例输出：

[ 9] Writing to out_file1...
[ 6] Writing to out_file2...
[ 5] Writing to out_file2...
[ 8] Writing to out_file1...
[ 4] Writing to out_file2...
[ 7] Writing to out_file1...
[ 3] Writing to out_file2...
[ 6] Writing to out_file1...
[ 2] Writing to out_file2...
[ 5] Writing to out_file1...
[ 1] Writing to out_file2...
[ 4] Writing to out_file1...
[ 8] Writing to out_file3...
[ 3] Writing to out_file1...
[ 7] Writing to out_file3...
[ 2] Writing to out_file1...
[ 6] Writing to out_file3...
[ 1] Writing to out_file1...
[ 5] Writing to out_file3...
[ 4] Writing to out_file3...
[ 3] Writing to out_file3...
[ 2] Writing to out_file3...
[ 1] Writing to out_file3...

Answer 2

这说明了示例代码的可运行版本：

import time
import threading

def export_data(fileName):
    # runs the db2 database command to export tables to file
    while True:
        print 'If I were the real function, I would be writing to ' + fileName
        time.sleep(1)

thread1 = threading.Thread(target=export_data, args=[ 'out_file1' ])
thread2 = threading.Thread(target=export_data, args=[ 'out_file2' ])

thread1.start()
thread2.start()

thread1.join()
thread2.join()

Answer 3

您的可见代码很好，但是我们看不到某些代码使用锁定，即使在数据库本身也可能发生锁定。

并发执行多个线程

3 个答案: