Exiting out of for loop and returning when at the end of a file

Question

I want to loop through a file and return every line:

for i in master_file:
        columns = re.split('\s{2,}', i)
        last_name = columns[1]
        print(last_name)

If I replace print with return it will only return columns[1] for the first line, but I want to return it for all the lines. So I can use that info elsewhere in my function.

return will terminate your loop, you need a different approach to this particular request like yield — e.doroskevic, Apr 05 '14 at 22:21

score 9 · Accepted Answer · answered Apr 05 '14 at 22:20

9

In this case you need to yield the value and not return it:

for i in master_file:
        columns = re.split('\s{2,}', i)
        last_name = columns[1]
        yield last_name

Complete sample:

def readLastNames ():
    for i in master_file:
            columns = re.split('\s{2,}', i)
            last_name = columns[1]
            yield last_name

for last_name in readLastNames ():
    print (last_name)

Very rough explanation: yield passes the parameters and the control back to the calling context, perserves the state of the called context and resumes then from there.

answered Apr 05 '14 at 22:20

Hyperboreus

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I've tried using yield before, I keep getting this error: `` – Amon Apr 05 '14 at 22:28
@Amon This is not an error. It is a generator. You need to consume it. Something like the last two lines of my second snippet. `for x in myGeneratorFunction(): print(x)`. – Hyperboreus Apr 05 '14 at 22:29
Oh my bad, it worked indeed. Can I use this method to create variables (like `last_name`) that I can use inside the function? – Amon Apr 05 '14 at 22:38
What name (variable) to assign to each yielded value is your choice. – Hyperboreus Apr 05 '14 at 22:54
So I have to use something like those last two lines right? What I want is to have different variables represent columns[1], columns[2] etc.. Then do something else with that variable, rather than just print it out – Amon Apr 05 '14 at 23:31
@Amon Process it as normal: `for x in generator(): doWhatEverYouWantWith (x)` – Hyperboreus Apr 06 '14 at 01:52

score 3 · Answer 2 · edited May 23 '17 at 11:49

3

Instead of return, use yield

It's like a return but it keeps the position in the iterable.

edited May 23 '17 at 11:49

Community

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answered Apr 05 '14 at 22:21

Savir

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score 1 · Answer 3 · answered Apr 05 '14 at 22:27

1

You could use a list comprehension,

return [re.split('\s{2,}', line, 2)[1] for line in master_file]

answered Apr 05 '14 at 22:27

Hugh Bothwell

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Exiting out of for loop and returning when at the end of a file

3 Answers3