Problems with a python function that return a list of strings?

Question

I have a set of files in a directory. So I created a function that apply some processing to all the files in the directory:

def fancy_function(directory, regex):
    for set the path of the directory:
       with open the file names and walk over them:
           preprocessing1 = [....]
           preprocessing2 = [ remove some punctuation from preprocessing1]

           return preprocessing2

Then I do the following:

list_of_lists = fancy_function(a_directory, a_regex)
print list_of_lists
>>>['processed string']

It just return one list and the directory actually have 5 files, then when I do the following:

def fancy_function(directory, regex):
    do preprocessing...
    preprocessing1 = [....]
    preprocessing2 = [ remove some punctuation from preprocessing1]
    print preprocessing2

print fancy_function(a_directory, a_regex)

It returns the 5 preprocessed files that I want like this:

['file one']
['file two']
['file three']
['file four']
['file five']

Why is this happening and how can I obtain the 5 files in a list?. I would like to save them In one list in order to make a nother processing but now for each list in the main list, something like this:

main_list =[['file one'], ['file two'], ['file three'], ['file four'], ['file five']]

I'm guessing you have the return inside a loop. return causes the function to exit, aborting any loops. Please show your actual code. — BrenBarn
– BrenBarn, Commented Jan 2, 2015 at 4:12
It's a lot of code.. I apply the return out of the for loop and now it just return 2 words of the final list. — newWithPython
– newWithPython, Commented Jan 2, 2015 at 4:21
you don't need yield... you need to append to the list - currently you are recreating the list each iteration. then you should also put the return outside the loop — Anentropic
– Anentropic, Commented Jan 2, 2015 at 4:27
@PauloScardine when I used yield it return the following: <generator object fancy_function at 0x10f2bd4b0>. How can I return the 5 lists in a list?. — newWithPython
– newWithPython, Commented Jan 2, 2015 at 4:30

Community · Accepted Answer · 2017-05-23 11:50:56Z

3

You have a return statement inside a for loop, which is a common gotcha. The function ends immediately, returning a single element, instead of returning a list of all the processed elements.

You have two options. First, you can explicitly define a list within your function, append intermediate results to that list, and return the list at the end.

def fancy_function(directory, regex):
    preprocessed_list = []
    for set the path of the directory:
        with open the file names and walk over them:
            preprocessing1 = [....]
            preprocessing2 = [ remove some punctuation from preprocessing1]

            preprocessed_list.append(preprocessing2)
    return preprocessed_list

Or fancier, you can turn your function into a generator.

def fancy_function(directory, regex):
    preprocessed_list = []
    for set the path of the directory:
        with open the file names and walk over them:
            preprocessing1 = [....]
            preprocessing2 = [ remove some punctuation from preprocessing1]

            yield preprocessing2 # notice yield, not return

This generator can then be used thus:

>>> preprocessed = fancy_function(a_directory, a_regex)
>>> print list(preprocessed)
[['file one'], ['file two'], ['file three'], ['file four'], ['file five']]

edited May 23, 2017 at 11:50

CommunityBot

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answered Jan 2, 2015 at 4:30

zehnpaard

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4 Comments

zehnpaard Over a year ago

Cool, no problem! Once you're comfortable with Python's basics, I really recommend reading up on generators, because my explanation above doesn't do it justice - them and the list comprehension family really add a bit of magic to Python's power and expressiveness.

newWithPython Over a year ago

Could you provide me some reference?.Thanks

zehnpaard Over a year ago

I added a link to a SO question about what generators are good for. The Python Wiki has a very quick intro on why and how you create a generator, I feel Jeff Knupp's blog post gives a more friendly discussion but you should stop when he starts talking about Moar Power. Once you're comfortable and want something truly mind-bending, read/watch David Beazley's presentations - example

zehnpaard Over a year ago

btw I didn't mean to say Jeff Knupp's post was weaker after Moar Power, it's just that the discussion goes towards coroutines, which uses the same syntax and mechanism, but adds another magnitude of complexity and conceptually has a very different use to standard generators. Once you've mastered generators and want more mind-warping programming concepts, you should go read up on coroutines. You'll probably encounter use cases for generators all the time, coroutines not so much.

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