This page is from the equivalent page in the excellent introduction to Python by Eric Matthes. The original page has an MIT license.

Introducing Functions

One of the core principles of any programming language is, “Don’t Repeat Yourself”. If you have an action that should occur many times, you can define that action once and then call that code whenever you need to carry out that action.

We are already repeating ourselves in our code, so this is a good time to introduce simple functions. Functions mean less work for us as programmers, and effective use of functions results in code that is less error-prone.

What are functions?

Functions are a set of actions that we group together, and give a name to. You have already used a number of functions from the core Python language, such as string.title() and list.sort(). We can define our own functions, which allows us to “teach” Python new behavior.

General Syntax

A general function looks something like this:

# Let's define a function.
def function_name(argument_1, argument_2):
    # Do whatever we want this function to do,
    # using argument_1 and argument_2
    ...

We would call the function like this:

# Use function_name to call the function.
function_name(value_1, value_2)

This code isn’t very useful, but it shows how functions are used in general.

• Defining a function
  • Give the keyword def, which tells Python that you are about to define a function.
  • Give your function a name. A variable name tells you what kind of value the variable contains; a function name should tell you what the function does.
  • Give names for each value the function needs in order to do its work.
    • These are basically variable names, but they are only used in the function.
    • They can be different names than what you use in the rest of your program.
    • These are called the function’s arguments.
  • Make sure the function definition line ends with a colon.
  • Inside the function, write whatever code you need to make the function do its work.
• Using your function
  • To call your function, write its name followed by parentheses.
  • Inside the parentheses, give the values you want the function to work with.
    • These can be variables such as current_name and current_age, or they can be actual values such as “eric” and 5.

Basic Example

For a simple first example, we will look at a program that compliments people. Let’s look at the example, and then try to understand the code. First we will look at a version of this program as we would have written it earlier, with no functions.

print("You are doing good work, Adriana!")
print("Thank you very much for your efforts on this project.")
print()  # Print a blank line.

print("You are doing good work, Billy!")
print("Thank you very much for your efforts on this project.")
print()

print("You are doing good work, Caroline!")
print("Thank you very much for your efforts on this project.")
print()

You are doing good work, Adriana!
Thank you very much for your efforts on this project.

You are doing good work, Billy!
Thank you very much for your efforts on this project.

You are doing good work, Caroline!
Thank you very much for your efforts on this project.

Functions take repeated code, put it in one place, and then you call that code when you want to use it. Here’s what the same program looks like with a function.

def thank_you(name):
    # This function prints a two-line personalized thank you message.
    print("You are doing good work, " + name + "!")
    print("Thank you very much for your efforts on this project.")
    print()  # Print a blank line.

thank_you('Adriana')
thank_you('Billy')
thank_you('Caroline')

You are doing good work, Adriana!
Thank you very much for your efforts on this project.

You are doing good work, Billy!
Thank you very much for your efforts on this project.

You are doing good work, Caroline!
Thank you very much for your efforts on this project.

In our original code, each pair of print statements was run three times, and the only difference was the name of the person being thanked. When you see repetition like this, you can usually make your program more efficient by defining a function.

The keyword def tells Python that we are about to define a function. We give our function a name, thank_you() in this case. A variable’s name should tell us what kind of information it holds; a function’s name should tell us what the variable does. We then put parentheses. Inside these parentheses we create variable names for any variable the function will need to be given in order to do its job. In this case the function will need a name to include in the thank you message. The variable name will hold the value that is passed into the function thank_you().

To use a function we give the function’s name, and then put any values the function needs in order to do its work. In this case we call the function three times, each time passing it a different name.

A common error

A function must be defined before you use it in your program or notebook. For example, putting the function at the end of the program or notebook cell would not work.

thank_you_effusively('Adriana')
thank_you_effusively('Billy')
thank_you_effusively('Caroline')

def thank_you_effusively(name):
    # This function prints another two-line personalized thank you message.
    print("You are doing EXCELLENT work, " + name + "!")
    print("Thank you very very much for your efforts on this project.")
    print()  # Print a blank line.

NameError                                 Traceback (most recent call last)
   ...
NameError: name 'thank_you_effusively' is not defined

On the first line we ask Python to run the function thank_you_effusively(), but Python does not yet know how to do this function. We define our functions at the beginning of our programs, and then we can use them when we need to.

Advantages of using functions

You might be able to see some advantages of using functions, through this example:

• We write a set of instructions once. We save some work in this simple example, and we save even more work in larger programs.
• When our function works, we don’t have to worry about that code anymore. Every time you repeat code in your program, you introduce an opportunity to make a mistake. Writing a function means there is one place to fix mistakes, and when those bugs are fixed, we can be confident that this function will continue to work correctly.
• We can modify our function’s behavior, and that change takes effect every time the function is called. This is much better than deciding we need some new behavior, and then having to change code in many different places in our program.

You can think of functions as a way to “teach” Python some new behavior. In this case, we taught Python how to say thank you to someone; now we can tell Python to do this with any name we choose, whenever we want to.