Python return list from function

Question

I have a function that parses a file into a list. I'm trying to return that list so I can use it in other functions.

def splitNet():
    network = []
    for line in open("/home/tom/Dropbox/CN/Python/CW2/network.txt","r").readlines():
        line = line.replace("\r\n", "")
        line = string.split(line, ',')
        line = map(int, line)
        network.append(line)
    return network

When I try to print the list outside of the function (for debugging) I get this error:

NameError: name 'network' is not defined

Is there something simple I am doing wrong or is there a better way to pass variables between functions without using globals?

Can you add the code where you assign network back to a variable in scope and print it? — sberry
– sberry, Commented Feb 16, 2012 at 18:36
your splitNet() function looks alright. Could you post the code tht prints the netwark-list? — alex
– alex, Commented Feb 16, 2012 at 18:37
If you want to pass variables between functions: why not using parameters? Why avoid globals? You can also put your function as methods and save the result inside the class instance no ? — tito
– tito, Commented Feb 16, 2012 at 18:37
I'm guessing you've found the problem judging from what you said because I don't do anything like that. At the moment I am just calling the function so it runs and then calling print network — Thomas Mitchell
– Thomas Mitchell, Commented Feb 16, 2012 at 18:38
Hi, i see you reverted my edit. I would argue, you should provide code that illustrates the problem and can be reproducible. Right now it is not complete. This might help: stackoverflow.com/help/mcve — aaaaa says reinstate Monica
– aaaaa says reinstate Monica, Commented Apr 9, 2019 at 15:50

Alex Coplan · Accepted Answer · 2012-02-16 18:37:50Z

65

Variables cannot be accessed outside the scope of a function they were defined in.

Simply do this:

network = splitNet()
print network

answered Feb 16, 2012 at 18:37

Alex Coplan

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3 Comments

Thomas Mitchell Over a year ago

Thanks. Man do I feel dumb now haha.

Alex Coplan Over a year ago

@Schmooo don't worry, I did this a while back :)

gag123 Over a year ago

Won't the list 'network' (created inside the function) be destroyed once the function execution ends ? Won't it be erased from the memory ? If it's erased, how can we access outside the function.

Alex Coplan · Accepted Answer · 2012-02-16 18:45:00Z

18

I assume you are not assigning the returned value to a variable in scope.

ie. you can't do

splitNet()
print network

instead you would

network = splitNet()
print network

or for that matter

my_returned_network_in_scope = splitNet()
print my_returned_network_in_scope

otherwise you could declare network outside of the splitNet function, and make it global, but that is not the recommended approach.

edited Feb 16, 2012 at 18:45

Alex Coplan

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answered Feb 16, 2012 at 18:37

sberry

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2 Comments

sberry Over a year ago

@Alex: Thanks for the edit, yes a typo. Going back and forth from Java to Python to PHP to Go to Erlang to C++... messes me up something awful.

Alex Coplan Over a year ago

Know what you mean... I've just started with Python and I've started dropping them in my PHP/Objective-C now!

Community · Accepted Answer · 2017-05-23 11:55:10Z

8

The names of variables in a function are not visible outside, so you need to call your function like this:

networks = splitNet()
print(networks)

A couple of other notes:

You may want to convert your function to an iterator, using yield.
You don't need to call readlines; the function itself is an iterator.
Your function may be leaking the file handle. Use the with statement.
You can use str.split, which is more readable and easier to understand than string.split.
Your file looks to be a CSV file. Use the csv module.

In summary, this is how your code should look like:

import csv
def splitNet():
    with open("/home/tom/Dropbox/CN/Python/CW2/network.txt") as nf:
        for line in csv.reader(nf, delimiter=','):
            yield map(int, line)
network = list(splitNet())
print (network)

edited May 23, 2017 at 11:55

CommunityBot

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answered Feb 16, 2012 at 18:37

phihag

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Comments

RanRag · Accepted Answer · 2012-02-16 18:38:59Z

6

Your function is returning a list so you have to assign it to a variable and than try to print it.

network = splitNet()
print network

For example

>>> def mylist():
...    myl = []
...    myl.append('1')
...    return myl
...
>>> my_list = mylist()
>>> my_list
['1']
>>>

answered Feb 16, 2012 at 18:38

RanRag

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Comments

Chris Taylor · Accepted Answer · 2012-02-16 18:38:10Z

2

Have you actually called the function yet? This works fine (in the Python interpreter)

 >>> def f():
 ...   network = []
 ...   network.append(1)
 ...   network.append(2)
 ...   network.append(3)
 ...   return network
 ...
 >>> network = f()
 >>> print network
 [1, 2, 3]

answered Feb 16, 2012 at 18:38

Chris Taylor

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Comments

Victor · Accepted Answer · 2016-04-28 18:42:53Z

1

You may declare the name of the variable assigned to the list as global, like this:

def get_list():
    global destination_list
    destination_list = []
    destination_list.extend(('1','2','3'))
    return destination_list

get_list()
print(destination_list)

answered Apr 28, 2016 at 18:42

Victor

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1 Comment

mbomb007 Over a year ago

True, but not the best way. Since the value is returned, it should be assigned rather than put in the global scope.

olinox14 · Accepted Answer · 2019-06-27 14:37:04Z

1

L=[1, 2, 3]

def rl(l): 
    return l

[*ll] = rl(L) # ll is in a list

ll
# >>> [1, 2, 3]

*t, = rl(L)   # ll is in a tuple

t
# >>> [1, 2, 3]

edited Jun 27, 2019 at 14:37

olinox14

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answered Jun 27, 2019 at 12:30

Rathinavelu Muthaliar

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Comments

Lord Pantaloon · Accepted Answer · 2018-10-07 06:08:39Z

0

If you want to return an item or list from a definition, you could define it before hand and use it as a variable during the initial writing of said definition. Unless it has to be defined within the definition. In this case you won't need to write in a return command at the end.

network = []

def splitNet(network):
    for line in open("/home/tom/Dropbox/CN/Python/CW2/network.txt","r").readlines():
        line = line.replace("\r\n", "")
        line = string.split(line, ',')
        line = map(int, line)
        network.append(line)

print network # Will print the list you've appended. But it is now a usable object.

answered Oct 7, 2018 at 6:08

Lord Pantaloon

1

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