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Python Python Basics (Retired) Putting the "Fun" Back in "Function" Functions

Posted by Jan M
Jan M
1,053 Points

Can someone please walk me through this question? I have no Idea what to do even once presented with the answer :/

I am talking about this question:

"Make a function named add_list that takes a list. The function should then add all of the items in the list together and return the total. Assume the list contains only numbers. You'll probably want to use a for loop. You will not need to use input()."

I watched all the Videos till now and felt 'quite' confident, however I noticed that I always struggled with the Code Challenges.. are they supposed to be this hard or was I just not paying attention?

Could someone please explain this question and the answer to me?

Thanks alot

functions.py 
# add_list([1, 2, 3]) should return 6
# summarize([1, 2, 3]) should return "The sum of [1, 2, 3] is 6."
# Note: both functions will only take *one* argument each.

4 Answers

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

Let's walk through it. Given the problem:

Make a function named add_list that takes a list. The function should then add all of the items in the list together and return the total. Assume the list contains only numbers. You'll probably want to use a for loop. You will not need to use input().

Step 1. Make a function named add_list that takes a list.

This means "[Define] a function named add_list that takes a list [as an argument].". A simple function can be defined as below. Note the argument can be any name you wish. It will be assigned the list when the function is called. It is typical to choose a name that best represents what the variable will contain. pass is a keyword and is used as a placeholder for a block of code. pass does nothing.

def add_list(lst):
    pass

Step 2. The function should then add all of the items in the list together....

Adding all of the items in the list will require a loop. (there are other ways but for beginning coders learning about loops is important). Replacing pass with our loop:

def add_list(lst):
    # loop over every item in lst
    for item in lst:
        # add this item to the running total of items
        total = total + item

The above code is not complete. The issue is total doesn't have a initial value. This will cause an error the first time through the loop. Fix by adding initialization to total:

def add_list(lst):
    # initialize total
    total = 0
    # loop over every item in lst
    for item in lst:
        # add this item to the running total of items
        total = total + item

Step 3. ... and return the total. Including a return statement is crucial. without the return the function's result is set to None by default.

def add_list(lst):
    # initialize total
    total = 0
    # loop over every item in lst
    for item in lst:
        # add this item to the running total of items
        total = total + item
    # return the total
    return total

Note that he return statement indentation is the same as the for loop statement. A common beginner mistake is indenting the return too much which makes it part of the for loop code block. This would cause the function to return after the first pass through the loop.

Jan M
Jan M
1,053 Points

Thanks a lot for the quick and detailed reply, it really helped me understand this stuff!

Mohamed THIAM
Mohamed THIAM
1,091 Points

Hi Chris,

Thank you. This is super helpful. I'm still confused though as to:

  • where "item" comes from and what is its nature.
  • as a beginner when can I know that I need to assign a starting value to "total" for the operation to run? Is it a default procedure anytime I want to +/-* something to another item? I don't understand the logic here..

Cheers,

Mohamed.

Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

Hi Mohamed,

Where "item" comes from and what is its nature

item comes from the for statement. In this statement, for takes each element of the iterable (in this case, the passed in lst) one at a time and assigns it to the local variable item (this can be any variable name) then executes the code block following the for statement. So the loop will run for each of the elements in the iterable lst. For example

for number in range(5):
     print(number)

Range returns an iterable (0, 1, 2, 3, 4). The for loop will run 5 times, with number assigned to the next element in the iterable.

When can I know that I need to assign a starting value to "total" for the operation to run? Is it a default procedure anytime I want to +/-* something to another item? I don't understand the logic here..

In any statement, the variables must be defined before they are referenced. When using a += or -=, these are the equivalent of having the results variable (in this case total on both sides of the equation. So on the first loop, when the right-side of the statement is evaluated, total will not yet have a value. There are two ways this can be resolved:

# First way is using a 'try/except' statement:
def add_list(lst):
    # loop over every item in lst
    for item in lst:
        # try to add item to total
        try:
            # add this item to the running total of items
            total = total + item
        except NameError:
            # total not yet defined
            # initialize total to the value of item
            total = item
    # return the total
    return total

# But this that is much more complicated.
# Since we know that for the first loop, total needs a value
# the easy answer is to just start it with 0:
def add_list(lst):
    # initialize total
    total = 0
    # loop over every item in lst
    for item in lst:
        # add this item to the running total of items
        total = total + item
    # return the total
    return total

So, yes, every time you use a +=, -=, etc. you will need to initialize the variable somewhere above that statement. This is also true if you use the long form:

total = total + item

Mohamed THIAM
Mohamed THIAM
1,091 Points

Chris, I though I had it figured out but then this happened. Can you tell me the difference between these two pieces? Both of them work but the logic sort of escapes me.

<p>
def add_list(list):
  total = 0
  for item in list
    total += item
  return total
</p>

<p>
def add_list(list):
  for item in list
    total = 3
    total += item
  return total
</p>

If I understand correctly, in "total = 3", 3 refers to the 3 list items. But those were never defined. Or was it an implicit variable of the challenge?

Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

Mohamed,

The second solution is passing because of guessing the correct answer the challenge is looking for. In each loop, total is reset to 3, then incremented by the value of item. At the end of the loop the total will always be "3 + last item in list".

def add_list(list):
    for item in list
        total = 3  # <-- reset total to 3
        total += item  # <-- add current item to total
    return total  # <-- total will always be 3 + last item

This trivial code will also pass:

def add_list(arg1):
    return 6
Mohamed THIAM
Mohamed THIAM
1,091 Points

Thanks for being so helpful Chris! This is super awesome.

Chris Grazioli
Chris Grazioli
31,225 Points
Chris Grazioli
Chris Grazioli
31,225 Points

I think I'm having issues with the naming conventions and trying to pass in the argument "list" into the function the problem asks us to create

def add_list(list)
     for num in list:
         list_total+=num
         return list_total

I see that list gets highlighted red in Workspaces so its probably a keyword or something, but when I try to pass something else like a_list I seem to be running into the same sort of issue

Chris Grazioli
Chris Grazioli
31,225 Points
Chris Grazioli
Chris Grazioli
31,225 Points

since I figured out what I was missing, I have a new question, why aren't my forum posts being formatted... I wrapped the code in question with the "```" (triple back-ticks) and specified python, how is the MOD getting the cool formatting??? Backticks are the little guys under the tilde key right?

Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

There are two requirements for cool formatting:

  • Triple backtick: ```python

  • A blank link before and after the code block.

I added a blank line to your comment.

Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

The root issue is list_total is not initialized to 0. The statement

list_total += num is equivalent to list_total = list_total + num.

The first time this executes the right-side of the assignment has no value for list_total. Solve by adding

list_total = 0 before the for loop.

Chris Grazioli
Chris Grazioli
31,225 Points
Chris Grazioli
Chris Grazioli
31,225 Points

I seriously can't believe I'm screwing this up because of a missing colon " : "

Chris Grazioli
Chris Grazioli
31,225 Points
Chris Grazioli
Chris Grazioli
31,225 Points

Mohamed THIAM The item which should be named something else is the argument you are passing into the function. It can be called anything from "a_list_of_numbers" to "xyz" that part doesn't matter. You are telling the function def what the function will accept. In this case it's a list of numbers which the function will loop through and add to a local variable called list_total, it must be initialized to zero to start, or it will have no value.