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Python Python Basics (Retired) Putting the "Fun" Back in "Function" Shopping List Redux

André Rybakken
André Rybakken
459 Points
Posted by André Rybakken
André Rybakken
459 Points

Use of "Continue"

Is the Continue statement really necessary in the infinite while-loop?

while True:
  new_item = input("> ")
  if new_item == 'DONE':
    show_list()
    break
  add_to_list(new_item)
  continue

1 Answer

Ryan Ruscett
Ryan Ruscett
23,309 Points
Ryan Ruscett
Ryan Ruscett
23,309 Points

Hey,

You are right! The continue isn't necessary. Neither is the break. But it's good to know what they do. I only find em useful in very nested loop/if/elif type deals.

TIP! You need to ask yourself if a loop needs to be infinite. When I create a small server program. I will use While True, because I want the server to not be able to stop it. Although, in a program, you should leave a way out for the user. You should have a Boolean value like the one I used in this program for the exercise. So now I can turn it off whenever I type DONE - which is the second tip down below the code. 

shopping_list = list() ; print("Pick up at the store?\n Enter 'DONE' to stop.")
Run = True
while Run:
    new_item = raw_input("Item> ") 
    if new_item.upper() == 'DONE':
        print("List has {} items.\n your list {}".format(len(shopping_list), shopping_list))
        Run = False  
    shopping_list.append(new_item)
print("Added! List has {} items and here they are {}".format(len(shopping_list), shopping_list))

TIP - Think about if the user will actually type DONE DoNe or done. You still want it to exit properly. So add the .upper like I did above to do the check.

Does this answer your question? If so let me know, if not let me know how I can help clarify it more.

André Rybakken
André Rybakken
459 Points
André Rybakken
André Rybakken
459 Points

Thanks for the swift response!

Nicolas Hampton
Nicolas Hampton
44,638 Points

Just a note:

In Andre's copy of the code (sorry about the misspelling of your name) a continue or break is actually still necessary. If he removes it from his copy of the code the way it is, he'll end up with a shopping list that has 'DONE' as the last item, because add_to_list will run after the DONE conditional, adding that command to the list we're appending to. But continue isn't needed, you're right, because it's at the end of the loop already, so there's no code that we're trying to avoid running afterwards.

Also, in Ryan's copy of the code, if someone enters 'DONE' (or 'DonE', because they've accommodated for case-sensitive answers), five things will happen:

  1. The list will print

  2. Run will be changed to false, but the run condition won't be checked until the beginning of the next loop

  3. 'DONE' or 'DonE' will be appended to the end of the shopping list

  4. Run will then be checked and found to be false, thus breaking the loop

  5. The list will needlessly be printed again, this time with our 'DONE' command as the last item, as that print function is outside of our loop.

So you want something a little inbetween, to try and keep the code as dry as possible (non-repeatative) but still run well. Something like (and of course this might not be perfect, as I'm a beginner in python):

# we could also say 'shopping_list = []' here
shopping_list = list()  

# The '\n' is an escape character to make a new line
print("Pick up at the store?\n Enter 'DONE' to stop.")

Run = True

while Run:
    # By uppercasing all input, we can clean the list up a bit, 
    # but that's more a preference thing in this example
    new_item = raw_input("Item> ").upper()
    if new_item == 'DONE':
        Run = False
        # we need this continue to avoid appending 'DONE' to the end 
        # of the shopping list, jumping back to the Run test instead
        continue
    shopping_list.append(new_item)
# Now this is the only print we need to make for every scenerio
print("Added! List has {} items and here they are {}".format(len(shopping_list), shopping_list))

Because there's still code in the loop when we test for DONE, we still need some type of out. Using a boolean (the Run variable, booleans are True/False values) in the while loop instead of an infinite True loop means when we change Run to False, so we need to go back and run the while loop one more time to test the conditional, unlike the break, where we instantly break out of the loop. Just keep that in mind. And if you find yourself cutting and pasting a long print line like that, it probably means you can find a way of not doing that if you restructure the program. This is a good small exercise in refactoring code, and it was fun! Happy Coding!

Nicolas