10.3. Defining Our Own Functions¶

Built-in functions in Python give us a lot of capabilities. Indeed, we gain even more coding potential by writing our own functions.

We’ll start by looking at the general syntax.

10.3.1. Function Syntax¶

To create a function, use the following syntax:

def function_name( parameters ):
   # Code statements...

def is a keyword that instructs Python to create a new function.

Following def is the function name, which is the label that Python attaches to the code. When writing our own functions, the programmer chooses the function name. Like any other variable, a function name should describe the purpose of the function. The stronger we make the name, the less confusion we add to our code. A list of good naming practices appears in the next section.

Following the function name, we define parameters within the parentheses. Parameters are variables that can only be used within the function itself. We can define a function with one parameter, multiple parameters, or no parameters at all.

The number and names of the parameters depends on what we want the function to do. Parameters specify what information, if any, the function needs in order to do its job.

The def statement ends with a colon.

10.3.1.1. Naming Functions¶

Python function names should follow these rules:

Names CANNOT match any Python keyword like print or for.

Tip

Most code editors use syntax highlighting to indicate keywords. Paying attention to the highlighting helps avoid this naming mistake.
Names use only lowercase letters.
The name should describe exactly what the function does.

Tip

You should prefer long, descriptive names over short, abbreviated names. If someone unfamiliar with your code can read the function name and tell you what it does, then you chose a good name.
For names with multiple words, separate the words with underscores (e.g. convert_score_to_percentage).

Examples

Good

convert_feet_to_meters
is_valid_number_entry
draw_square

Not Good

convert
is_valid
draw

10.3.2. Function Code¶

After the def statement comes the function body. This is where we code the action that the function carries out. The function body can contain any amount of code (statements, loops, conditionals, etc.), but the lines must be indented when compared to the def keyword. Python recognizes the end of the function body once it finds the first unindented line after the def keyword.

Example

def add_list_numbers(list_name):
   total = 0
   for number in list_name:
      total += number

   return total

print("Hello, World!")

Line 1 defines the function name and parameter. Lines 2 - 6 are part of the function body. Line 8 is even with the def keyword, so it is NOT part of the add_list_numbers function.

10.3.3. Try It!¶

Let’s see function syntax in action. We first consider a loop that prints a list of names.

names = ["Lena", "James", "Julio"]
other_names = ["Devon", "Colin", "Nalini Nadkarni"]

for name in names:
   print(name)

As written, this loop only prints out the names from the first list. Let’s create a function that prints out the items from any list of names.

Example

Follow these steps to code your first function:

On line 2, use the def keyword to set the name of the function and the name of the parameter. Don’t forget to end the line with a colon.

def print_names(names_list):
On lines 3 and 4, use a for loop to print each name from names_list. Don’t forget to indent the lines compared to the def statement!
3 4
for name in names_list: print(name)
Click Run to make sure your code contains no syntax errors. If it does, debug the code before moving on.
The program will not actually print anything to the console yet, but we will fix that in a moment!

We can now use print_names just like the built-in Python functions. To call our function, we need to type its name and include a list as the argument.

In the editor above, call our new function twice by adding these lines below the function body:

print_names(names)
print('------')
print_names(other_names)

Run the code and examine the output. By calling the function with different arguments (names and other_names), we use the same loop to print the elements of each list to the console.

Note

There is nothing about our function that forces names_list to actually contain names, or even strings. The function will work the same for ANY list it is given. (TRY IT!)

Therefore, a better name for this function would be print_list_items!

10.3.4. Defining vs. Calling¶

When we define a function, we make it available for later use. However, a function does NOT run when it is defined. It must be called in order to execute.

Warning

This is not only a common point of confusion for new programmers, but it can also cause logic errors!

Let’s look at another example to see how this works.

Try It!

What happens if we define a function without calling it?

Run the following code as-is (the code contains no bugs). What gets printed?

In order for a function to run, it must be called by using its name. Add the code say_hello() in line 5 and run the program again.
Does it matter if the say_hello() statement is indented relative to the def statement? Try changing the indentation to check!
Does it matter if we include parentheses in line 5? Try using say_hello instead of say_hello().

10.3.5. Check Your Understanding¶

Question

What are the parameters of the following function? Click ALL that apply.

def draw_square(turtle_name, side_length):
   for side in range(4):
      turtle_name.forward(side_length)
      turtle_name.left(90)

bob = turtle.Turtle()
draw_square(bob, 100)

bob
turtle_name
side_length
side
100

Question

For the same code sample, what are the arguments sent by the function call? Click ALL that apply.

bob
turtle_name
side_length
side
100

Question

True or False: A function can be run several times by placing the function call inside a loop.

True
False

Question

Which is the BEST name for the following function?

def func_name(radius):
  area = 3.14159 * radius**2
  return area

area
calculate_area_of_circle
circle
shape