Python Functions: Arguments and Return Values

Python Functions at a Glance

A function is a named block of code that runs when called. It can receive data through parameters, perform one focused task, and optionally return a result.

1. Create and Call a Function

Start a function definition with def, choose a meaningful name, add parentheses and a colon, and indent the code that belongs to the function. The definition stores the work; the later call runs it.

def show_connection_message():
    print("We will attempt to establish a connection.")

show_connection_message()

We will attempt to establish a connection.

The name show_connection_message describes the function's job. The parentheses in the call are required even though this function accepts no data.

2. Use Arguments and Parameters

A function can receive strings, integers, floats, lists, tuples, dictionaries, or other Python objects. The variable named in the definition is a parameter; the value supplied in the call is an argument.

def connect_to_device(device_name):
    print(f"Connecting to device {device_name} initiated")

connect_to_device("SW1")

Connecting to device SW1 initiated

In this example, device_name is the parameter and "SW1" is the argument. Inside the function, the parameter refers to the supplied string.

3. Pass Multiple Arguments

Separate multiple parameters and arguments with commas. Positional arguments are matched from left to right, so their order matters.

def show_device_info(device_name, vlan):
    print(f"Device {device_name} uses VLAN: {vlan}")

show_device_info("SW1", 10)

Device SW1 uses VLAN: 10

4. Return Data from a Function

Printing displays a value for a person. Returning sends a value back to the calling code so the program can store, test, format, or pass it elsewhere.

def show_status_code(http_code):
    if http_code == 200:
        return "Request OK"
    elif http_code == 404:
        return "Not found"
    return "Unknown status"

status = show_status_code(200)
print(status)

Request OK

The fallback return ensures the function also produces a defined result for status codes other than 200 and 404. Without an explicit return, a Python function returns None.

5. Reduce Repeated Code

Functions are most useful when the same task would otherwise appear in several places. Instead of repeating three status messages for every device, put that behavior in one function and pass the changing device name as an argument.

def show_connection_messages(network_device_name):
    print(f"Attempt to connect to device {network_device_name} in progress.")
    print("Connection in progress")
    print("Connection successful!")

show_connection_messages("SW1")
show_connection_messages("SW2")

Attempt to connect to device SW1 in progress.
Connection in progress
Connection successful!
Attempt to connect to device SW2 in progress.
Connection in progress
Connection successful!

If the message format changes, you now update one function instead of finding and editing several repeated blocks.

6. Use a Function with a Loop

Combine a reusable function with a for loop to process several devices consistently. The loop chooses each item; the function owns the work performed for that item.

def show_connection_messages(device):
    print(f"Attempt to connect to device {device}")
    print("Connection in progress")
    print("Connection successful!\n")

routers = ["R1", "R2", "R3"]
switches = ["SW1", "SW2", "SW3"]

for router in routers:
    show_connection_messages(router)

for switch in switches:
    show_connection_messages(switch)

7. Conclusion

A function packages one task behind a useful name. Define it with def, call it with parentheses, pass changing data as arguments, and use return when the rest of the program needs a result. Small, focused functions reduce repetition and make network automation scripts easier to test, read, and maintain.