6.1 Defining functions

Learning objectives

By the end of this section you should be able to

• Identify function calls in a program.
• Define a parameterless function that outputs strings.
• Describe benefits of using functions.

Calling a function

Throughout the book, functions have been called to perform tasks. Ex: print() prints values, and sqrt() calculates the square root. A function is a named, reusable block of code that performs a task when called.

print("Please log in")
username = input("Username:")
password = input("Password:")
print("Login successful")
print("Welcome,", username)

# Use float() to convert input for area calculation
width = float(input("Enter width:"))
height = float(input("Enter height:"))
print("Area is", width*height)

Defining a function

A function is defined using the def keyword. The first line contains def followed by the function name (in snake case), parentheses (with any parameters—discussed later), and a colon. The indented body begins with a documentation string describing the function's task and contains the function statements. A function must be defined before the function is called.

def water_plant:

def print_phone_num():
  print("Phone: (", 864, ")", 555, "-", 0199)

print("User info:")
print_phone_num()

Benefits of functions

A function promotes modularity by putting code statements related to a single task in a separate group. The body of a function can be executed repeatedly with multiple function calls, so a function promotes reusability. Modular, reusable code is easier to modify and is shareable among programmers to avoid reinventing the wheel.