why does my itertools.combinations content disappear as soon as I start a loop?

Question

Here is my python codes for constructing image pairs from a list of images then loop through them:

from itertools import combinations
import os

root = "/root/code/set1/"
folders = "images"
os.chdir(root+folders)
with open(os.path.join(root, root.split("/")[-1]+"_results.txt"), 'w') as f:
    
    
    metrics = ["cosine", "euclidean"]
    print("starting the looop")
    jresults = {}
    
    images = os.listdir(os.path.join(root, folders))
    images = [image for image in images if image.endswith(".jpg")]

    image_pairs = combinations(images, 2)

    print("number of pairs: {}".format(len(list(image_pairs)))) ## here it prints out correct numbers like 21325546
    for metric in metrics:
        print("metric: {}".format(metric))
        print("number of pairs: {}".format(len(list(image_pairs)))) ## HERE IT PRINTS 0 AND I COULDN'T FIGURE OUT WHY
        for img1, img2 in image_pairs:
            print("img1: {}, img2: {}".format(img1, img2)) ## HERE PRINTS NOTHING

I simply couldn't figure out why it disappeared and nothing is there to be looped through. Could anyone kindly help? Thanks so much!

`list(image_pairs)` iterates over the iterator, the iterator is now exhausted, the next time you try to use it, it will be empty — juanpa.arrivillaga, Jun 29 '22 at 19:28

score 3 · Answer 1 · answered Jun 29 '22 at 19:20

3

You can make it a list to persist it:

image_pairs = list(combinations(images, 2))

combinations returns a generator, not a list in memory.

answered Jun 29 '22 at 19:20

JacobIRR

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1

To be pedantic, it isn't a generator, but it is an iterator – juanpa.arrivillaga Jun 29 '22 at 19:29

why does my itertools.combinations content disappear as soon as I start a loop?

1 Answers1