[SOLVED] Python: Passing variables between functions

Issue

I’ve spent the past few hours reading around in here and elsewhere, as well as experimenting, but I’m not really understanding what I am sure is a very basic concept: passing values (as variables) between different functions.

For example, I assign a whole bunch of values to a list in one function, then want to use that list in another function later:

list = []

def defineAList():
    list = ['1','2','3']
    print "For checking purposes: in defineAList, list is",list
    return list

def useTheList(list):
    print "For checking purposes: in useTheList, list is",list

def main():
    defineAList()
    useTheList(list)

main()

Based on my understanding of what function arguments do, I would expect this to do as follows:

1. Initialize ‘list’ as an empty list; call main (this, at least, I know I’ve got right…)
2. Within defineAList(), assign certain values into the list; then pass the new list back into main()
3. Within main(), call useTheList(list)
4. Since ‘list’ is included in the parameters of the useTheList function, I would expect that useTheList would now use the list as defined by defineAList(), NOT the empty list defined before calling main.

However, this is obviously a faulty understanding. My output is:

For checking purposes: in defineAList, list is ['1', '2', '3']
For checking purposes: in useTheList, list is []

So, since “return” obviously does not do what I think it does, or at least it does not do it the way I think it should… what does it actually do? Could you please show me, using this example, what I would have to do to take the list from defineAList() and use it within useTheList()? I tend to understand things better when I see them happening, but a lot of the examples of proper argument-passing I’ve seen also use code I’m not familiar with yet, and in the process of figuring out what’s going on, I’m not really getting a handle on this concept. I’m using 2.7.

ETA- in the past, asking a similar question, it was suggested that I use a global variable instead of just locals. If it will be relevant here also- for the purposes of the class I’m taking, we’re not permitted to use globals.

Thank you!

Solution

This is what is actually happening:

global_list = []

def defineAList():
    local_list = ['1','2','3']
    print "For checking purposes: in defineAList, list is", local_list 
    return local_list 

def useTheList(passed_list):
    print "For checking purposes: in useTheList, list is", passed_list

def main():
    # returned list is ignored
    returned_list = defineAList()   

    # passed_list inside useTheList is set to global_list
    useTheList(global_list) 

main()

This is what you want:

def defineAList():
    local_list = ['1','2','3']
    print "For checking purposes: in defineAList, list is", local_list 
    return local_list 

def useTheList(passed_list):
    print "For checking purposes: in useTheList, list is", passed_list

def main():
    # returned list is ignored
    returned_list = defineAList()   

    # passed_list inside useTheList is set to what is returned from defineAList
    useTheList(returned_list) 

main()

You can even skip the temporary returned_list and pass the returned value directly to useTheList:

def main():
    # passed_list inside useTheList is set to what is returned from defineAList
    useTheList(defineAList()) 

Answered By – Pavel Anossov

Answer Checked By – Mary Flores (BugsFixing Volunteer)