Multiple threads writing to the same CSV in Python

Question

I'm new to multi-threading in Python and am currently writing a script that appends to a csv file. If I was to have multiple threads submitted to an concurrent.futures.ThreadPoolExecutor that appends lines to a csv file. What could I do to guarantee thread safety if appending was the only file-related operation being done by these threads?

Simplified version of my code:

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)) 

And my thread class being:

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

                #Some code.....

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

Should I set up a seperate single-threaded executor to handle writing or is it woth worrying about if I am just appending?

EDIT: I should elaborate that when the write operations occur can vary greatly with minutes between when the file is next appended to, I am just concerned that this scenario has not occurred when testing my script and I would prefer to be covered for that.

Answer 1

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.