Multiple threads writing to the same CSV in Python

I am not sure if csvwriter is thread-safe. The documentation doesn't specify, so to be safe, if multiple threads use the same object, you should protect the usage with a 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()))

That being said, it may indeed be more elegant for the downloadThread to submit write tasks to an executor, instead of explicitly using locks like this.

Way-late-to-the-party note: You could handle this a different way with no locking by having a single writer consuming from a shared Queue, with rows being pushed to the Queue by the threads doing the processing.

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()

here is some code, it also handles the headache-causing unicode issue:

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'中文', 'bar'])

a more detailed solution is here