Introduction to functions


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Your First Function!

Below is the code for a function that adds all the numbers in a list and then sends back the sum. We will explore each piece of the code directly

def sum_numbers(lst): """ This is a docstring, and it describes your function INPUT: lst is a list that we want to sum across OUTPUT: s is a scalar that is the sum of the list """ # Declare the sum s = 0 # Iterate through each number in the list for number in lst: # And add the number to the sum each time s += number # Return the sum return s # Declare a new list new_list = [1,2,3,4,3,2,1] #Pass the list to the function we just created print('The sum of the numbers in the list! ',sum_numbers(new_list))

Ok so the first line def sum_numbers(lst): is the declaration of our function.

  • def refers to define and sum_numbers is the name of our function.

Functions then have parentheses which contain the arguments that are being passed to the function. In this case, we are just passing a list and therefore lst is inside the parentheses.

You may have a situation when you require no arguments and on the converse a situation where you require many arguments.

The next couple of lines should look familiar:

lst = [1,2,3,4,5] s = 0 for number in lst: s += number print(s)

This is just a for loop that iterates through the list.

Finally, at the end we have the statement return s. This is our return statement that sends back a value (or in some cases an object or list) back to where we called the function.

Once you have built the function, you can easily call it using many different lists.

To call the function, you must first define it, and then you can access it by running sum_numbers(list) in your python script

Also, don't forget to add your docstring so that someone else who wants to use your function can figure out what is going on!


  • What does sum_numbers return for the list [1,1,1,1,15,2,1,2,3]?
  • How about lst = range(100)?

Your Turn! Test this out in the terminal!

  • try turning this into a function
# Declare the list x = list(range(100)) #Declare a variable for the sum s = 0 #Iterate through the list for number in lst: #Put your code Here pass print(s)
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