ThreadPoolExecutor vs threading.Thread

Question

I've got a question regarding performance of ThreadPoolExecutor vs Thread class on its own which seems to me that I lack some fundamental understanding.

I've a web scraper in two functions. First to parse the links for each image of a website homepage and the second to load an image off the link parsed:

import threading
import urllib.request
from bs4 import BeautifulSoup as bs
import os
from concurrent.futures import ThreadPoolExecutor

path = r'C:\Users\MyDocuments\Pythom\Networking\bbc_images_scraper_test'
url = 'https://www.bbc.co.uk'

# Function to parse link anchors for images
def img_links_parser(url, links_list):
    res = urllib.request.urlopen(url)
    soup = bs(res,'lxml')
    content = soup.findAll('div',{'class':'top-story__image'})

    for i in content:
        try:
            link = i.attrs['style']
            # Pulling the anchor from parentheses
            link = link[link.find('(')+1 : link.find(')')]
            # Putting the anchor in the list of links
            links_list.append(link)
        except:
            # links might be under 'data-lazy' attribute w/o paranthesis
            links_list.append(i.attrs['data-lazy'])

# Function to load images from links
def img_loader(base_url, links_list, path_location):
    for link in links_list:
        try:
            # Pulling last element off the link which is name.jpg
            file_name = link.split('/')[-1]
            # Following the link and saving content in a given direcotory
            urllib.request.urlretrieve(urllib.parse.urljoin(base_url, link), 
            os.path.join(path_location, file_name))
        except:
            print('Error on {}'.format(urllib.parse.urljoin(base_url, link)))

The following code is split up in to two cases:

Case 1: I'm using multiple threads:

threads = []
t1 = threading.Thread(target = img_loader, args = (url, links[:10], path))
t2 = threading.Thread(target = img_loader, args = (url, links[10:20], path))
t3 = threading.Thread(target = img_loader, args = (url, links[20:30], path))
t4 = threading.Thread(target = img_loader, args = (url, links[30:40], path))
t5 = threading.Thread(target = img_loader, args = (url, links[40:50], path))
t6 = threading.Thread(target = img_loader, args = (url, links[50:], path))

threads.extend([t1,t2,t3,t4,t5,t6])
for t in threads:
    t.start()
for t in threads:
    t.join()

The above code does its job on my machine for 10 seconds.

Case 2: I'm using ThreadPoolExecutor

with ThreadPoolExecutor(50) as exec:
    results = exec.submit(img_loader, url, links, path)

The above code results to 18 seconds.

My understanding was that ThreadPoolExecutor creates a thread for each worker. So, given I set max_workers to 50 would result to 50 threads and therefore should have completed the job faster.

Can someone please explain what am I missing here? I admit that I'm making a silly mistake here but I just don't get it.

Many thanks!

Answer 1

In Case 2 you're sending all the links to one worker. Instead of

exec.submit(img_loader, url, links, path)

you'd need to:

for link in links:
    exec.submit(img_loader, url, [link], path)

I didn't try it out myself, that's just from reading the documentation of ThreadPoolExecutor