11.3. Defining Our Own Functions

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

We’ll start by looking at the general syntax.

11.3.1. 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. Naming Functions

Python function names should follow these rules:

  1. Names CANNOT match any Python keyword like print or for.


    Most code editors use syntax highlighting to indicate keywords. Paying attention to the highlighting helps avoid this naming mistake.

  2. Names use only lowercase letters.

  3. The name should describe exactly what the function does.


    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.

  4. For names with multiple words, separate the words with underscores (e.g. convert_score_to_percentage).



  • convert_feet_to_meters

  • is_valid_number_entry

  • draw_square

Not Good

  • convert

  • is_valid

  • draw

11.3.2. 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.


def add_numbers_together(num):
   total = 0
   while num < 100:
      total += number
      number +=1

   return total

print("Hello, World!")

Line 1 defines the function name and parameter. Lines 2 - 6 are part of the function body. Line 89 is even with the def keyword, so it is NOT part of the add_list_numbers function.

11.3.3. 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.


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

11.3.4. Check Your Understanding


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

def perimeter_of_square(side_length):
   return side_length * 4

  1. perimeter_of_square

  2. side_length

  3. print


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

  1. square

  2. side_length

  3. 4


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

  1. True

  2. False


Which is the BEST name for the following function?

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

  2. calculate_area_of_circle

  3. circle

  4. shape