Understanding compact code snippet

Question

I have inherited some Python scripts and I'm working to understand them. I am a beginner-level Python programmer but very experienced in several other scripting languages.

The following Python code snippet generates a file list which is then used in a later code block. I would like to understand exactly how it is doing it. I understand that os.path.isfile is a test for filetype and os.path.join combines the arguments in to a filepath string. Could someone help me understand the rest?

flist = [file for file in whls if os.path.isfile(os.path.join(whdir, i, file))]

things_we_want = [thing for thing in list_of_things if we_want(thing)] — Davidmh
– Davidmh, Commented Aug 6, 2014 at 22:29
@Davidmh that's possibly the most concise means of explanation I've ever seen.... — Adam Smith
– Adam Smith, Commented Aug 6, 2014 at 22:32

Adam Smith · Accepted Answer · 2014-08-06 22:30:33Z

1

whls is an iterable of some kind.

For each element in whls, it checks if os.path.join(whdir, i, that_element) is a file. (os.path.join("C:","users","adsmith") on Windows is r"C:\users\adsmith")

If so, it includes it in that list.

As @jonsharpe posted in the comments, this is an example of a list comprehension which are well worth your time to master.

answered Aug 6, 2014 at 22:30

Adam Smith

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Tom Dalton · Accepted Answer · 2014-08-06 22:30:59Z

0

The list comprehension means that python will iterate over each member of whls (this is maybe a tuple/list?), and for each item, it will test whether os.path.join(whdir, i, file) is a file (as opposed to a directory etc). It will return a list containing only the elements from whls that pass this condition check.

answered Aug 6, 2014 at 22:30

Tom Dalton

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ojy · Accepted Answer · 2014-08-07 00:16:20Z

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This list comprehension is equivalent to the following loop:

flist = []
for file in whls:
    if os.path.isfile(os.path.join(whdir, i, file)):
        flist.append(file)

The list comprehension is more compact. Performance-wise, they are similar, with the list comprehension being a little faster because it doesn't load the append() method.

answered Aug 7, 2014 at 0:16

ojy

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1 Comment

ddoug Over a year ago

Thank you for the alternate code. It made it much easier to understand. I will work on learning list comprehension.

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