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Python List Comprehensions Gone Crazy

Recently, I was messing around with some image recognition tasks in Python, and wanted to take a look at some of the images in my training set. At first, I was tempted to go open file explorer and walk around the directory tree spot-checking a few, but then I remembered that it was easy to display images in an IPython notebook using `IPython.display`.

Perhaps I could write some code to walk the tree and show a few random images from each directory. Surely it couldn't be that hard. I wrote a simple method to do so:

import os
import random
from IPython.display import display, Image

def select_rand(num, dirname, filenames):
    files = [file for file in filenames if file.endswith(('.jpg','.png'))]
    if len(files) > 0:
        print('Found ' + str(len(files)) + ' files in ' + dirname)
        files = random.sample(files, min(len(files), num))
        for filename in files:
            display(Image(os.path.join(dirname, filename)))
    else:
        print('Found no files for ' + dirname)

os.path.walk(root_dir, select_rand, 2)

Probably not the most efficient in the world, but it did exactly what I wanted. Of course, at this point I was basically procrastinating from what I was actually supposed to be doing, so decided to take it further.

Could I reduce this whole shebang to a single list comprehension? Is this good programming? Nope, definitely not, but it's a fun way to test your understanding of a language, so let's do it.

First, let's find a method that returns all of the files in a directory tree, walking the tree like `os.path.walk` but returning the results instead of visiting them - `os.walk` should do the trick. Why is `os.path.walk` a visitor pattern and `os.walk` a generator? No clue, let's move on.

[t for t in os.walk(root_dir)]

Second, we'll filter the files to images only:

[img
 for t in os.walk(root_dir)
 for img in t[2] if img.endswith(('.png', '.jpg'))]

Now let's turn these into actual Image objects by replacing `img` with `Image(os.path.join(t[0], img))`. Okay, we now have Image objects for all images, but that's too many to display in our notebook. We should limit to three random ones from each directory:

[Image(os.path.join(t[0], img))
 for t in os.walk(root_dir) 
 for img in random.sample([f for f in t[2] if f.endswith(('.png','.jpg'))], 3)]

We replace our `t[2]` with a random sample based on an inner comprehension, and move our "is it an image" test into that comprehension. This, however, explodes in my example with

ValueError: sample larger than population

Boom! I apparently have some subdirectory in there with only one or two images. Rather than try for some even crazier nested inner lambda, I'll just define a simple specialization of `sample` that's protected against too few results, and replace `random.sample` with that. We then `map` each result to the `display` method to show it in the notebook:

sample = lambda l, n: l if len(l) < n else random.sample(l, n)
map(display, [Image(os.path.join(t[0], img))
 for t in os.walk(root_dir) 
 for img in sample([f for f in t[2] if f.endswith(('.png','.jpg'))], 3)])

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