python list comprehension does not work with list.extend

Question

I have a list of glob patterns. For each glob pattern, I get the matching file list, and I want to combine all the file lists with list comprehension to be one flattened list:

patterns = [
    "some_pattern1",
    "some_pattern2"
]
all_files = []
all_files.extend(glob.glob(pattern)) for pattern in patterns

However this gives me a syntax error, I can do the list merge with:

for pattern in patterns:
    all_files.extend(glob.glob(pattern))

I think those two syntax are equivalent. Why the list comprehension doesn't work with extend?

For exmaple, consider some_pattern1 matches two files [1.txt, 2.txt]; some_patter2 matches [3.txt, 4.txt], I'm looking for a way to combine the two lists to be: [1.txt, 2.txt, 3.txt, 4.txt]

I think you should use `all_files.extend([glob.glob(pattern) for pattern in patterns])`. — Yang Hanlin, Mar 30 '20 at 05:05
Could you please give us an example of what you are trying to achieve — chintan thakrar, Mar 30 '20 at 05:07
@YangHanlin sorry didn't make myself clear, I wanted to have a flattened list which is a result of the combined list. Your answer gives me (in the updated example: `[[[1.txt,2.txt],[3.txt,4.txt]]]`) — Chen Xie, Mar 30 '20 at 05:25
Does https://stackoverflow.com/questions/952914/how-to-make-a-flat-list-out-of-list-of-lists help? — Karl Knechtel, Mar 30 '20 at 05:45

Dhaval Taunk · Accepted Answer · 2020-03-30T05:46:19.697

1

There is no problem with the extend method.

Your syntax is wrong. You should write it this way:-

patterns = [
    "some_pattern1",
    "some_pattern2"
]
all_files = []
all_files.extend([glob.glob(pattern) for pattern in patterns])

It will return output like this:-

[['1.txt', '2.txt'], ['3.txt', '4.txt']]

Add this to the code:

sum(all_files, [])

Now the output will be like this:-

['1.txt', '2.txt', '3.txt', '4.txt']

edited Mar 30 '20 at 05:46

answered Mar 30 '20 at 05:40

Dhaval Taunk

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Never use `sum` for list concatenation. Doing so takes quadratic time. – user2357112 Apr 05 '20 at 06:10

score 0 · Answer 2 · answered Mar 30 '20 at 05:11

0

This is simply invalid syntax, all_files.extend(glob.glob(pattern)) for pattern in patterns looks like the syntax for list comprehension, which you're not trying to perform here.

answered Mar 30 '20 at 05:11

Sush

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python list comprehension does not work with list.extend

2 Answers2