Parameters and Return Values

Introduction

Parameters allow you to pass data into a function. Return values allow a function to send data back to the caller. Together they make functions truly reusable.

Parameters

def greet(name):
    print(f"Hello, {name}!")

greet("Alice")   # Hello, Alice!
greet("Bob")     # Hello, Bob!

Multiple Parameters

def describe_employee(firstname, department, salary):
    print(f"{firstname} works in {department} and earns £{salary:,}")

describe_employee("Alice", "Engineering", 80000)

Default Parameter Values

If a caller does not provide a value, the default is used.

def greet(name, greeting="Hello"):
    print(f"{greeting}, {name}!")

greet("Alice")              # Hello, Alice!
greet("Bob", "Good morning")  # Good morning, Bob!

Default parameters must come after non-default ones.

Keyword Arguments

You can pass arguments by name, in any order.

def describe_employee(firstname, department, salary):
    print(f"{firstname} — {department} — £{salary:,}")

describe_employee(salary=80000, firstname="Alice", department="Engineering")

Return Values

The return statement exits the function and sends a value back to the caller.

def add(a, b):
    return a + b

result = add(10, 5)
print(result)   # 15

Without a return, a function returns None.

Returning Multiple Values

Python functions can return multiple values as a tuple.

def get_name_and_salary(employee_id):
    # In a real program, this might query a database
    return "Alice", 80000

name, salary = get_name_and_salary(1)
print(name, salary)   # Alice 80000

*args and **kwargs

For functions that accept a variable number of arguments.

# *args collects extra positional arguments as a tuple
def total(*amounts):
    return sum(amounts)

print(total(100, 200, 300))   # 600

# **kwargs collects extra keyword arguments as a dictionary
def display(**details):
    for key, value in details.items():
        print(f"{key}: {value}")

display(name="Alice", department="Engineering", salary=80000)

Practice Exercises

• Write a function calculate_tax(salary, rate=0.2) that returns the tax amount. Test with and without providing the rate.
• Write a function full_name(firstname, lastname) that returns the full name as a single string.
• Write a function that takes any number of salaries using *args and returns the average.
• Write a function employee_summary that accepts keyword arguments and prints each key-value pair.