variable defined in a function throws NameError: global name not defined when used in main

Question

I'm trying to run my function: show_total() , but I get this error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".\file.py", in <module>
    main()
  File ".\file.py", in main
    total_money = show_total(bags, coins, coin_to_bag)
NameError: global name 'coin_to_bag' is not defined

my code looks like:

def assign_coin_to_bag(bags, coins):
    coin_to_bag = {}
    print(bags)
    print('\n')
    print (coins)
    print('\n')
    for bag in bags:
        print('For bag: ' + bag )
        coin_type = input('\nWhich coin do you want to put in this bag? ')  #e.g. 0.25 * 2
        coin_amount = input('\nNumber of this type? ')  #e.g. 0.25 * 2
        mattress_status = 'stuffed'
        for coin in coins:
            coin_to_bag[bag] = [coin_type, coin_amount, mattress_status]
    print(coin_to_bag)
    return (coin_to_bag)

def main():
    bags = gather_bag()
    coins = gather_coin()
    coins_in_the_bag = assign_coin_to_bag(bags, coins)
    total_money = show_total(bags, coins, coin_to_bag)

main()

Thank you for your help!

Please post your main function as well. It seems like the coin_to_bag variable does not exist at the time of calling show_total from main — ffledgling
– ffledgling, Commented Dec 7, 2015 at 16:22
It doesn't help to see the function body when the error occurs where you're passing in the argument. — Two-Bit Alchemist
– Two-Bit Alchemist, Commented Dec 7, 2015 at 16:23
in show_total() your program will crash if user enters *. You cannot change * to int. if int(single_or_all_total) == 1: — matejm
– matejm, Commented Dec 7, 2015 at 16:30

Kristján · Accepted Answer · 2015-12-07 16:52:40Z

3

coin_to_bag is a defined in the scope of assign_coin_to_bag, and is not accessible in main. You caught the return value of assign_coin_to_bag in coins_in_the_bag, and need to use that variable in your call to show_total.

There's a common error new programmers make, which is thinking the name of a variable needs to be the same wherever it's used, even across methods. In fact, the name of the variable means absolutely nothing to the computer. Good names are only there for humans.

As an exercise, we used to have students trace code like this:

def foo(one, three, two):
  print "%s %s %s" % (one, two, three)

def bar():
  return 1, 2, 4

three, two, one = bar()
foo(two, one, three)

Figure out what gets printed, and that will be good practice to break the variable naming habit.

edited Dec 7, 2015 at 16:52

answered Dec 7, 2015 at 16:33

Kristján

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4 Comments

Kristján Over a year ago

The same way you've been using variables everywhere else. show_total(bags, coins, coins_in_the_bag).

Jshee Over a year ago

Does both the declaration need to change to: def show_total(bags,coins,coins_in_the_bag): ??? and the call: total_money = show_total(bags, coins, coin_to_bag,coins_in_the_bag) ?

Kristján Over a year ago

No. Your confusion here is that you think a variable must be named the same thing everywhere in the program - that would make writing and using libraries impossible. The name of the variable you pass does not have to be the same as the name of the argument it's received in, nor the same as the return value it caught.

Jshee Over a year ago

Great catch, and thanks for helping me to understand my error.

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