多个线程在Python中写入相同的CSV

Question

我是Python的多线程新手,目前正在编写一个附加到csv文件的脚本.如果我要将多个线程提交给一个concurrent.futures.ThreadPoolExecutor将行附加到csv文件的行.如果附加是这些线程唯一与文件相关的操作,我该怎么做才能保证线程安全？

我的代码的简化版本:

with concurrent.futures.ThreadPoolExecutor(max_workers=3) as executor:
    for count,ad_id in enumerate(advertisers):

        downloadFutures.append(executor.submit(downloadThread, arguments.....))
        time.sleep(random.randint(1,3)) 

我的线程类是:

def downloadThread(arguments......):

                #Some code.....

                writer.writerow(re.split(',', line.decode()))

我应该设置一个单独的单线程执行程序来处理写入,还是担心我是否只是附加？

编辑:我应该详细说明,当写入操作发生时,下一次附加文件之间的分钟数差别很大,我只关心在测试我的脚本时没有发生这种情况,我宁愿为此加以覆盖.

Answer 1

我不确定是否csvwriter是线程安全的.该文件没有明确规定,因此是安全的,如果多个线程使用同一个对象,你应该保护的使用有threading.Lock:

# create the lock
import threading
csv_writer_lock = threading.Lock()

def downloadThread(arguments......):
    # pass csv_writer_lock somehow
    # Note: use csv_writer_lock on *any* access
    # Some code.....
    with csv_writer_lock:
        writer.writerow(re.split(',', line.decode()))

话虽这么说,downloadThread向执行程序提交写入任务可能确实更优雅,而不是明确地使用这样的锁.

Answer 2

Way-late-to-the-party注意：您可以通过让单个写入器从共享队列中消费，并通过执行处理的线程将行推送到队列来以不同的方式处理此问题，而无需锁定。

from threading import Thread
from queue import Queue
from concurrent.futures import ThreadPoolExecutor


# CSV writer setup goes here

queue = Queue()


def consume():
    while True:
        if not queue.empty():
            i = queue.get()
            
            # Row comes out of queue; CSV writing goes here
            
            print(i)
            if i == 4999:
                return


consumer = Thread(target=consume)
consumer.setDaemon(True)
consumer.start()


def produce(i):
    # Data processing goes here; row goes into queue
    queue.put(i)


with ThreadPoolExecutor(max_workers=10) as executor:
    for i in range(5000):
        executor.submit(produce, i)

consumer.join()

Answer 3

这是一些代码，它还处理了令人头疼的 unicode 问题：

def ensure_bytes(s):
    return s.encode('utf-8') if isinstance(s, unicode) else s

class ThreadSafeWriter(object):
'''
>>> from StringIO import StringIO
>>> f = StringIO()
>>> wtr = ThreadSafeWriter(f)
>>> wtr.writerow(['a', 'b'])
>>> f.getvalue() == "a,b\\r\\n"
True
'''

    def __init__(self, *args, **kwargs):
        self._writer = csv.writer(*args, **kwargs)
        self._lock = threading.Lock()

    def _encode(self, row):
        return [ensure_bytes(cell) for cell in row]

    def writerow(self, row):
        row = self._encode(row)
        with self._lock:
            return self._writer.writerow(row)

    def writerows(self, rows):
        rows = (self._encode(row) for row in rows)
        with self._lock:
            return self._writer.writerows(rows)

# example:
with open('some.csv', 'w') as f:
    writer = ThreadSafeWriter(f)
    writer.write([u'??', 'bar'])

更详细的解决方案在这里