Thumbnail generator

Let us start our discussion of multi-threading in Python with the example of a program used to generate thumbnails of image URLs.

In the example, we are using Pillow, a fork of the Python Imaging Library (PIL) to perform this operation:

# thumbnail_converter.py
from PIL import Image
import urllib.request

def thumbnail_image(url, size=(64, 64), format='.png'):
    """ Save thumbnail of an image URL """

    im = Image.open(urllib.request.urlopen(url))
    # filename is last part of the URL minus extension + '.format'
    pieces = url.split('/')
    filename = ''.join((pieces[-2],'_',pieces[-1].split('.')[0],'_thumb',format))
    im.thumbnail(size, Image.ANTIALIAS)
    im.save(filename)  
    print('Saved',filename)

The preceding code works very well for single URLs.

Let us say we want to convert five image URLs to their thumbnails:

img_urls = ['https://dummyimage.com/256x256/000/fff.jpg',
            'https://dummyimage.com/320x240/fff/00.jpg',
            'https://dummyimage...