Can anyone help me understand, How the below code works?

Question

Part of code that I do not understand, how it work?

data_folders = [os.path.join(root, d) for d in sorted(os.listdir(root))
if os.path.isdir(os.path.join(root, d))]

Here is the link for the code: https://github.com/rndbrtrnd/udacity-deep-learning/blob/master/1_notmnist.ipynb

Screenshot for the entire code:

Question: "os.path.join(root, d)" joins the path and folder name (d) which is coming from the for loop, one at a time. But I do not understand what is the use of "if" condition which is doing the same thing? (I guess)

Please post all code as text. Screenshots of code are not acceptable on Stack Overflow. — Klaus D., Mar 14 '21 at 05:28
@the23Effect It's a called a *list comprehension*, there's also a generator expression with `( )` around it. — Klaus D., Mar 14 '21 at 05:30
"But I do not understand what is the use of "if" condition which is doing the same thing? (I guess)" It's not. it checks if the path created by `os.path.join` is a directory. — juanpa.arrivillaga, Mar 14 '21 at 05:30
@KlausD. My bad initially the code did not show the square brackets and I saw the whole code later and forgot to change generator to comprehension. — the23Effect, Mar 14 '21 at 05:33
@KlausD. I wanted to highlight the part of the code that I did not understand and make it easy to observe, hence the image. I have also given the link to the entire code in the post. Thank you for helping out. — Kasid Khan, Mar 14 '21 at 05:33

score 1 · Answer 1 · answered Mar 14 '21 at 05:21

1

I read it like this:

Join path and folder name which is coming from the for loop one at a time, if the joined path and folder name is a directory. If not, just skip the said path.

The purpose of the code is to build a list of sorted directories.

answered Mar 14 '21 at 05:21

Nicholas D

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score 1 · Accepted Answer · answered Mar 14 '21 at 05:31

This is a list comprehension syntax. The internal part is actually a generator but the square bracket makes it as a list comprehension.

It contains three parts, and last part is optional.

output_list = [
    generate something                  # Part 1: this will be the output content.
    from some iterable                  # Part 2: this is where you pick elements from.
    which matches a condition           # Part 3: this is the optional condition to match.
]

So for your question this is how it should be read,

data_folders = [
    os.path.join(root, d)                       # Part 1: generate this.
    for d in sorted(os.listdir(root))           # Part 2: from these.
    if os.path.isdir(os.path.join(root, d))     # Part 3: if they meet this condition.
]

This is an awesome explanation. Thank you. – Kasid Khan Mar 14 '21 at 05:36 — Kasid Khan, Mar 14 '21 at 05:36

Can anyone help me understand, How the below code works?

2 Answers2