lambda function and regular expression

Question

I am using a script from https://towardsdatascience.com/a-keras-pipeline-for-image-segmentation-part-1-6515a421157d to split a data set. I don't understand what this part is doing

all_frames = os.listdir(FRAME_PATH)
all_masks = os.listdir(MASK_PATH)


all_frames.sort(key=lambda var:[int(x) if x.isdigit() else x 
                                for x in re.findall(r'[^0-9]|[0-9]+', var)])
all_masks.sort(key=lambda var:[int(x) if x.isdigit() else x 
                               for x in re.findall(r'[^0-9]|[0-9]+', var)])

More specifically I do not understand what the everything the var: is doing. My first guess would be a list comprehension, but it does not follow the structure.

[ expression for item in list if conditional ]

Also what is the purpose of this part re.findall(r'[^0-9]|[0-9]+', var) ?

thank you

You can learn more about list comprehension syntax in this post, namely about if/else: stackoverflow.com/questions/4260280/… — Luka Mesaric
– Luka Mesaric, Commented May 25, 2020 at 16:22

David Robinson · Accepted Answer · 2020-05-25 16:39:01Z

2

The int(x) if x.isdigit() else x is a ternary operator ("if condition then this else that"), which you're right isn't part of the list comprehension. This is saying "turn x (from within the list comprehension) into an integer if it contains only digits".

So we could write this all out like:

def convert_integer(x):
    if x.isdigit():
        return int(x)
    else:
        return x

def key_function(var):
    return [convert_integer(x) 
               for x in re.findall(r'[^0-9]|[0-9]+', var)]

all_frames.sort(key = key_function)

edited May 25, 2020 at 16:39

answered May 25, 2020 at 16:21

David Robinson

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6 Comments

Lis Lou Over a year ago

yes, as I mentioned, what I dont understand is what the return part is doing

David Robinson Over a year ago

@LisLou Do you mean the ternary operator int(x) if x.isdigit() else x? I've added a section to my answer describing that as well

Luka Mesaric Over a year ago

@DavidRobinson Regarding x.isdigit(), given regex will match 'a', 'b' and 'c' in "abc" because of the first part [^0-9], so not everything has to be made up of digits.

Lis Lou Over a year ago

So, [int(x) if x.isdigit() else x for x in re.findall(r'[^0-9]|[0-9]+', var)] is a list comprehension statement where ` [ expression for item in list if conditional ]` expression is int(x) if x.isdigit() else x and there is no conditional at the end? only a for iteration over the elements of the list?

Lis Lou Over a year ago

I am not sure if I understand re.findall(r'[^0-9]|[0-9]+', var)], I tried this var = "blabla0236abs7b8b9b0102b1" print(re.findall(r'[^0-9]|[0-9]+', var)) and the output was ['b', 'l', 'a', 'b', 'l', 'a', '0236', 'a', 'b', 's', '7', 'b', '8', 'b', '9', 'b', '0102', 'b', '1'] From what I understand it will return a list of strings with all the matches of sequences of numbers[0-9], or this pattern [^0-9], according to the documentation ^ matches the start of a string, so [^0-9] means a sequence like (any character)+a number from 0 to 9?

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