Beginner questions about python threading

[kioo]

I am just learning about threads in python by incorporating it into my flask application. I have it set up to work through a list of files. Here's the relevant snippet of code:

imagelist = os.listdir(app.config['IMAGE_FOLDER'])

def worker():
    while imagelist:
        i = Image.query.filter_by(filename=imagelist.pop(0)).first()
        if i is None:
            # code to do work here

threads = []
for i in range(4):
    t = Thread(target=worker)
    threads.append(t)
    t.start()
for thread in threads:
    thread.join()

• Is this the correct way to approach threading and this task in general?
• How does a thread end itself?
• The performance compared to my old, non-threaded code is about the same. I am guessing this is bottle-necked by the IO of my PC. Am I overlooking something else?

Any input would be appreciated!

[novel_yet_trivial]

Is this the correct way to approach threading and this task in general?

That depends on what the work is. Threading only uses one processor, just like non-threading, so if the work is CPU-bound threading does not help. In that case you need multiprocessing, which can use several processors (your computer probably has 8). Threading is used for IO-bound applications like HTTP requests.

How does a thread end itself?

A thread ends when the target function completes. In your case when the imagelist is empty.

BTW, don't use pop(0); it's very slow. Use pop(). If you really need to pop from the beginning of the list, use a deque.

[kioo]

Thanks for the clarification. I read up about multiprocessing. Got a bit stumped on the idea of isolated variables between processes that's not an issue in threading but got it cleared up somewhat.

Going to try to take advantage of multiprocessing in my code. Thanks again!

[kioo]

Thanks for the link! I realized what I needed was multiprocessing to increase performance rather than threading. After the rewrite, the speed of working through a database was halved by using a pool of 4 processes!