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Python

Posted by Valerio Versace
Valerio Versace
11,319 Points

Shopping List extra credit

Hi, I'm trying to do the Python Basics shopping list extra credit. I'm having a problem with what I assume is Python variable scope.

If I do the program this way, it works:

def show_help():
  print("What do you want to buy?")
  print("""
Enter 'DONE' to stop adding items
Enter 'HELP' for this help.
Enter 'SHOW' to see your current list.
Enter 'SAVE' to save the list to a file (list.txt in the current folder).
""")

def print_list():
  print("Here's your list:")
  for item in shopping_list:
    print(item)

def add_to_list(new_item):
  shopping_list.append(new_item)
  print("Added {}. Shopping list now has {} items.".format(new_item, len(shopping_list)))

def save_list():
  with open("shopping_list", "w") as textfile:
    for item in shopping_list:
      textfile.write(item + "\n")
  print("List saved!")

#BEGIN

shopping_list = open("shopping_list").read().splitlines()

load_list()

show_help()

while True:
  new_item = input("> ")
  if new_item == "DONE":
    break
  elif new_item == "HELP":
    show_help()
  elif new_item == "SHOW":
    print_list()
  elif new_item == "SAVE":
    save_list()
  else:
    add_to_list(new_item)

if len(shopping_list) > 0:
  print_list()

However, if I refactor the code and declare shopping_list as an empty list in the global scope and then assign it a value via a function that I make, it does not work. shopping_list is not assigned any value.

Here's the refactored code which does not work:

def show_help():
  print("What do you want to buy?")
  print("""
Enter 'DONE' to stop adding items
Enter 'HELP' for this help.
Enter 'SHOW' to see your current list.
Enter 'SAVE' to save the list to a file (list.txt in the current folder).
""")

def print_list():
  print("Here's your list:")
  for item in shopping_list:
    print(item)

def add_to_list(new_item):
  shopping_list.append(new_item)
  print("Added {}. Shopping list now has {} items.".format(new_item, len(shopping_list)))

def load_list():
  with open("shopping_list") as textfile:
    shopping_list = textfile.read().splitlines()

def save_list():
  with open("shopping_list", "w") as textfile:
    for item in shopping_list:
      textfile.write(item + "\n")
  print("List saved!")

#BEGIN

shopping_list = []

load_list()

show_help()

while True:
  new_item = input("> ")
  if new_item == "DONE":
    break
  elif new_item == "HELP":
    show_help()
  elif new_item == "SHOW":
    print_list()
  elif new_item == "SAVE":
    save_list()
  else:
    add_to_list(new_item)

if len(shopping_list) > 0:
  print_list()

Any idea why, and how I could fix it (or possibly make it more elegant)? Thank you!

1 Answer

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,468 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,468 Points

The difference is in referencing vs. assigning a global variable.

While you can access global variables without the global keyword, if you want to assign to them you have to use the global keyword. For example:

foo = 1
def test_creates_local():
    foo = 2 # new local foo

def test_modifies_global():
    global foo
    foo = 3 # changes the value of the global foo

So you have two choices:

  • add the global shopping_list declaration inside of your functions, or
  • pass shopping_list as an argument

To pass as an argument, use a mutable type:

>>> shopping_list = []
>>> def test_modifies_global(shopping_list, new_item):
...     shopping_list.append(new_item)  # changes the value of the global shopping_list
...     print("shopping_list", shopping_list)
... 
>>> test_modifies_global(shopping_list, 'apples')
shopping_list ['apples']
>>> test_modifies_global(shopping_list, 'apples')
shopping_list ['apples', 'apples']
>>> shopping_list
['apples', 'apples']
Kevin Faust
Kevin Faust
15,353 Points

I've read that the use of global variables is bad practice. Is this true?

Chris Freeman
Chris Freeman
Treehouse Moderator 68,468 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,468 Points

Yes. Passing an argument should take precedent over using a global variable, because "explicit is always better than implicit". In this case, it helps in debugging by keeping the scopes of the variables clear.

For more checkout this StackOverflow: why are global variables evil

EDIT: modified answer above to provide better examples.

Valerio Versace
Valerio Versace
11,319 Points

Thanks, it works perfectly now! So even inside functions I can always reference global variables, but never assign them unless I pass them to the function inside a parameter or reference them inside the function as "global".

Is this the case for most programming languages? I feel like this is often implied but not stressed enough.

Chris Freeman
Chris Freeman
Treehouse Moderator 68,468 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,468 Points

The topic is called variable scope and the exact rules vary slightly for each language but the global vs local concepts are generally true.