CODEWITHSUNDEEP
pythonpython-3.4
Registered User
Registered User

Reputation: 474

Python: Returning a list doesn't work

I am trying to make a program that can add/delete/show students in a class, and the 5 classes are 5 lists in a list.

Help is greatly appreciated.

When I run this code:

global classes
def intro():
    print("Welcome to Powerschool v2.0!")
    print("Actions:")
    print("1. Add Student")
    print("2. Delete Student")
    print("3. Show Students in a Class")
    print("4. Show All Students")
    x = int(input())
    while x<1 or x>4:
        print ("Please choose an action, 1-4.")
        x = int(input())
    if x == 1:
        action1()
    elif x == 2:
        action2()
    elif x == 3:
        action3()
    elif x == 4:
        action4()
    classes = [[],[],[],[],[]]
    return classes
def action1():
    print("Which Class? 1-5")
    a = int(input())
    print("Please enter the student's name.")
    z = input()
    classes[a-1].append(z)
    again()
def action2():
    print ("Which Class? 1-5")
    print ("Which student?")
    again()
def action3():
    print ("Which Class? 1-5")
    y = int(input())
    if y == 1:
        print (classes[0])
    elif y == 2:
        print (classes[1])
    elif y == 3:
        print (classes[2])
    elif y == 4:
        print (classes[3])
    elif y == 5:
        print (classes[4])
    again()
def action4():
    print (classes)
    again()
def again():
    print("Would you like to do something else? y/n")
    h = input()
    if h == "y":
        intro()
    else:
        quit
def main():

    intro()

main()

My error is:

Traceback (most recent call last):
  File "C:\Documents and Settings\user1\My Documents\Downloads\az_studenttracker.py", line 67, in <module>
    main()
  File "C:\Documents and Settings\user1\My Documents\Downloads\az_studenttracker.py", line 65, in main
    intro()
  File "C:\Documents and Settings\user1\My Documents\Downloads\az_studenttracker.py", line 19, in intro
    action1()
  File "C:\Documents and Settings\user1\My Documents\Downloads\az_studenttracker.py", line 33, in action1
    classes[a-1].append(z)
NameError: name 'classes' is not defined

I did return classes at the end of intro() but I see that doesn't work. I followed some suggestions, and nothing really happened :/

Upvotes: 1

Views: 221

Answers (4)

Asad-ullah Khan
Asad-ullah Khan

Reputation: 1833

This is because classes is out of scope for the second two methods. Therefore, you have two options:

Option 1

Pass classes to the methods action1(), action2(), etc like so:

def action1(classes)

...and the when you call it:

action1(classes) //with the classes var you just made

Option 2 (recommended)

Simply put the classes var outside your methods or declare it global like so:

global classes = [[],[],[],[],[]]

...right before:

def intro()

In general, you should read up on how return works; it is not necessary in the code you wrote

Upvotes: 1

Jason Baker
Jason Baker

Reputation: 2481

return doesn't do what you think it does. return statements are a way of passing execution control back up a context (For example, from intro() to main()), with the ability to send back some information for the higher context to use. Although you're passing classes back to main(), you never do anything with it at that context so it goes away.

One way to solve the problem would be to declare classes as a global variable. This is the easiest thing to do, but isn't generally good design. You could do this either by using the global keyword before declaring the local variable classes in intro() (See this question for guidance on global), or by declaring classes outside any of your functions.

Another solution would be to pass classes as a parameter to your action functions.

In either case, you would need to declare classes before any calls to your action functions.

Upvotes: 2

Padraic Cunningham
Padraic Cunningham

Reputation: 180391

classes only exists in intro():, you would have to declare it as a global variable to access it in other functions or declare it outside the function.

classes = [[],[],[],[],[]] # can be accessed by action3() ,action4()
def intro():

Upvotes: 1

Celeo
Celeo

Reputation: 5682

You're defining classes in your intro method, and, even though it's returning it, your action1 method doesn't see any variable named classes anywhere.

Relevant answer on Python scope and relevant documentation.

Upvotes: 5

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