Function Basics


Creating a Function

A simple function in Python uses the following structure:

def function_name():
    <block of statements>

Let’s break this structure down to see how it works:

  1. First, a function definition in Python begins with the word def, which is short for define. In effect, we are stating that we are defining a new function using the special def keyword.
  2. Next, we include the name of the function. Function names in Python follow the same rules as variable names:
  3. A function name must begin with a letter or underscore _. Function names beginning with an underscore _ have special meaning, so we won’t use those right now.
  4. A function name may only include letters, numbers, and underscores _.
  5. After the function name, we see a set of parentheses (). This is where we can add the parameters for the function. We’ll see how to add function parameters later in this lab.
  6. Then, we end the first line of a function definition in Python with a colon :. This tells us that the block of statements below the function definition is what should be executed when we run the function. We’ve already seen colons in both conditional statements and loops, and here it is used in a similar way.
  7. Finally, we have the block of statements that are part of the function itself. This block of statements must be indented one level to indicate that it is part of the function and not something else. When we reach the end of the function, we cease to indent the code, which tells Python that we’ve ended the function and are starting something else. So, notice that there is no closing curly brace } or any other marker to indicate the end of the function - only the indentation tells us where the function’s code ends. Again, this is very similar to how conditional statements and loops are structured in Python.

Function Example

Let’s look at an actual function in Python. In this case, we’ll create a simple hello_world function in Python:

def hello_world():
    print("Hello World")

Really, this structure is exactly like we’d expect it to be, based on the structures we’ve learned so far in Python. We start with the def keyword, then the name of the function. After that, we have parentheses () where the parameters would go, and then a colon : to show the start of the block of statements. Inside of the function, we indent the block of statements that make up the code to be run when this function is run - in this case we just print the string "Hello World" to the terminal. That’s really all there is to it!

Calling a Function

Likewise, calling a function in Python is exactly the same as what we’ve seen with the print() function - we simply state the name of the function, followed by a set of parentheses where we would place the arguments if the function required any parameters:


So, a complete Python program that includes the hello_world function and calls that function would look like this:

def hello_world():
    print("Hello World")


Try placing this code in a Python file and see if you can run it. It should work!

Notice that the function call for hello_world is not indented, and we included a couple of blank lines after the end of the definition of that function before the next piece of code. We do this for two reasons:

  1. We do not indent code that is not part of the block of statements within a function. Since the function call for hello_world should not be part of the definition for the function, we don’t want to indent it.
  2. In Python, the convention is to include two blank lines at the end of a function definition. So, that is why we included two blank lines in the code. These blank lines are not required, but it is good practice to include them.

Code Tracing a Function Call

Finally, let’s briefly go through an example of tracing a program that includes a couple of functions and function calls. Here’s a quick example we can use:

def foo():

def bar():


To trace this example, copy-paste the code into Python Tutor, or click this Python Tutor link.

When the program first loads, we should see this setup:

Python Tutor 1 Python Tutor 1

Again, this is very familiar to us based on what we’ve seen so far. So, let’s click the Next > button to step through the first part of the code:

Python Tutor 2 Python Tutor 2

As we can see, the first thing that Python sees is the definition for the foo function. Instead of keeping a separate list of functions, Python Tutor simply adds that function to the global frame just like any variable. The function itself is added to the Objects list, and then there is an arrow, connecting the variable in the global frame to the function in the objects list. In fact, this is exactly how Python handles functions - they are just treated like variables that are pointers to the function objects themselves! So, when we click Next > again, we’ll see it do the same to the bar function as well:

Python Tutor 3 Python Tutor 3

At this point, we are ready to execute the first function call, which will run the foo function. So, when we click the Next > button, we’ll see the arrow jump to the start of that function:

Python Tutor 4 Python Tutor 4

Here, we see Python tutor do something a bit different than in our own code traces - instead of keeping all of the variables in a single box, it creates a new frame to keep track of the variables that are created in the foo function. When we look an example of a function with parameters, we’ll see where this becomes very useful. For now, we can just ignore it and move on by clicking Next > once again:

Python Tutor 5 Python Tutor 5

At this point, the function is ready to print the string value "eye" to the terminal. So, if we click Next > once again, we should see that reflected in the output of Python Tutor:

Python Tutor 6 Python Tutor 6

At this point, we’ve reached the end of the foo function. So, Python Tutor will remove the foo frame from the list of frames and it will jump back down to where the function was called from when we click Next >:

Python Tutor 7 Python Tutor 7

Here, we are back to the main piece of our code, and we’re ready to call the bar function. So, we can click Next > once again:

Python Tutor 8 Python Tutor 8

This will jump up to the start of the bar function and create a new frame for storing any variables created by bar. We can click Next > again to enter the function:

Python Tutor 9 Python Tutor 9

And then click Next > again to print the string value "to" to the output:

Python Tutor 10 Python Tutor 10

Once again, we are at the end of a function, so when we click Next > once more:

Python Tutor 11 Python Tutor 11

We’ll move back to the main piece of the code, removing the frame for bar. On this line, we see yet another function call, this time to the foo function. Hopefully by this point we can predict what is going to happen, so we can click Next > a few more times to jump to the foo function and execute it. When it is done, we’ll see this output:

Python Tutor 15 Python Tutor 15

There we go! We’ve successfully traced the execution of a Python program that contains multiple function calls. The full process is shown in this animation:

Python Tutor Animation Python Tutor Animation

Using tools like Python Tutor to trace through code is a great way to make sure we understand exactly what our computer is doing when it is running our code.