Functions

A function is a named, reusable block of code. Instead of copying the same logic in multiple places, you define it once and call it whenever you need it.

Defining and Calling a Function

Use the def keyword, followed by a name, parentheses, and a colon. The function body is indented below:

# Define the function
def greet():
    print("Hello, Network Engineer!")

# Call the function
greet()   # → Hello, Network Engineer!

Functions with Parameters

Parameters are the variables listed in the function definition. They act as placeholders — when you call the function, you pass in the actual values (called arguments):

def add_device(network_devices, device):
    network_devices.add(device)
    return network_devices

# Call with arguments
my_devices = {"Switch", "Router", "Firewall"}
my_devices = add_device(my_devices, "Load Balancer")
# my_devices now contains "Load Balancer" too

The key insight: parameters are flexible. The function doesn't care which specific variable you pass — it just works on whatever is given to it. This is what makes functions reusable:

def configure_devices(device_list):
    for device in device_list:
        print(f"Configuring {device}")

routers = ["Router1", "Router2"]
switches = ["Switch1", "Switch2"]

configure_devices(routers)    # works on routers
configure_devices(switches)   # same function, different data

Returning a Value

The return statement sends a result back to wherever the function was called. Without return, the function returns None by default:

def count_devices(network_devices):
    return len(network_devices)

devices = ["Switch", "Router", "Firewall"]
count = count_devices(devices)
print(count)  # 3

Default Parameter Values

You can set a default value for a parameter. If the caller doesn't provide that argument, the default is used:

def connect(ip, port=22):
    print(f"Connecting to {ip} on port {port}")

connect("192.168.1.1")          # Uses default: port 22
connect("192.168.1.1", 8080)    # Overrides default: port 8080

pass — Placeholder for Empty Functions

If you need to define a function but aren't ready to write the body yet, use pass to avoid a SyntaxError:

def my_function():
    pass    # valid empty function — does nothing

Why Functions Matter

• Reusability — Write the logic once, use it anywhere
• Readability — A meaningful function name explains what the code does without reading every line
• Modularity — Break large problems into small, focused pieces
• Less repetition — Change the logic in one place instead of hunting it down everywhere