Python Passing variables between functions

Question

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    Initialize  list  as an empty list  call main  this  at least  I know I ve got right     Within defineAList    assign certain values into the list  then pass the new list back into main    Within main    call useTheList list  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

User · Answer

passing variable from one function as argument to other functions can be done like this

define functions like this

def function1():
global a
a=input("Enter any number\t")

def function2(argument):
print ("this is the entered number - ",argument)

call the functions like this

function1()
function2(a)

User · Answer

Your return is useless if you don t assign it  list defineAList

User · Answer

return returns a value   It doesn t matter what name you gave to that value   Returning it just  passes it out  so that something else can use it   If you want to use it  you have to grab it from outside   lst   defineAList   useTheList lst    Returning list from inside defineAList doesn t mean  make it so the whole rest of the program can use that variable    It means  pass this variable out and give the rest of the program one chance to grab it and use it    You need to assign that value to something outside the function in order to make use of it   Also  because of this  there is no need to define your list ahead of time with list        Inside defineAList  you create a new list and return it  this list has no relationship to the one you defined with list      at the beginning   Incidentally  I changed your variable name from list to lst   It s not a good idea to use list as a variable name because that is already the name of a built-in Python type   If you make your own variable called list  you won t be able to access the builtin one anymore

User · Answer

Read up the concept of a name space  When you assign a variable in a function  you only assign it in the namespace of this function  But clearly you want to use it between all functions    def defineAList         list     1   2   3   this creates a new list  named list in the current namespace       same name  different list       list extend  1    2    3    4    this uses a method of the existing list  which is in an outer namespace     print  For checking purposes  in defineAList  list is  list     return list   Alternatively  you can pass it around   def main        new list   defineAList       useTheList new list

User · Answer

You re just missing one critical step   You have to explicitly pass the return value in to the second function   def main        l   defineAList       useTheList l    Alternatively   def main        useTheList defineAList      Or  though you shouldn t do this  It might seem nice at first  but globals just cause you grief in the long run     l       def defineAList        global l     l extend   1   2   3     def main        global l     defineAList       useTheList l    The function returns a value  but it doesn t create the symbol in any sort of global namespace as your code assumes   You have to actually capture the return value in the calling scope and then use it for subsequent operations

User · Answer

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

[python] Python: Passing variables between functions

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