10 - Functions

Functions let you name and reuse a piece of logic.

A simple picture

A function is like a small machine with a name:

arguments -> function machine -> returned result

A machine may receive values through parameters, perform one clear job, and send a result back with return.

Defining and calling

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

message = greet("Asha")
print(message)

The def block creates the function. The call greet("Asha") runs it. name is a parameter; "Asha" is an argument.

Four common function types

We can group beginner functions by two questions:

Does the function receive input through parameters?
Does the function send a result back with return?

These are teaching categories, not four different Python keywords.

Type 1: No parameters and no return value

def show_welcome():
    print("Welcome to Python")

show_welcome()

This function always performs the same action. It does not receive data and does not send a result back.

Type 2: Parameters and no return value

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

greet("Asha")

The function receives name, but its job is only to display a message.

Type 3: No parameters and a return value

def get_course_name():
    return "Python Basic"

course_name = get_course_name()
print(course_name)

The function needs no input, but it sends a value back to the caller.

Type 4: Parameters and a return value

def add(first, second):
    return first + second

total = add(10, 20)
print(total)

This is the most reusable type because it accepts data and produces a result.

`return` versus `print`

print() displays a value. return sends a value to the caller:

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

total = add(2, 3)

Use return when another part of the program needs to use the result. A function without return returns None.

Important fact: print() displays something for a person. return sends a value back to the code that called the function. Printing a value does not automatically make it the function’s returned result.

Python can return more than one value. It actually packs the values into a tuple:

def calculate(a, b):
    return a + b, a - b

addition, difference = calculate(10, 3)
print(addition, difference)

Built-in and user-defined functions

Python provides built-in functions such as print(), len(), type(), sum(), and round(). You can call them without defining them first.

When you write a function with def, it is a user-defined function:

def double(number):
    return number * 2

Use a built-in function when it clearly solves the problem. Write your own function when you need project-specific behavior or want to give a repeated process a meaningful name.

Parameters, arguments, and defaults

def calculate_total(price, quantity, discount=0):
    subtotal = price * quantity
    return subtotal - subtotal * discount

print(calculate_total(100, 2))
print(calculate_total(100, 2, discount=0.10))

Positional arguments follow order. Keyword arguments name the parameter and make calls clearer. Required parameters should come before default parameters.

def introduce(name, city="Unknown"):
    return f"{name} lives in {city}"

print(introduce("Ravi"))
print(introduce("Ravi", city="Pune"))

Here name is required and city has a default value. A default is used only when the caller does not provide that argument.

Optional extension: variable-length arguments with *args and **kwargs

Variable-length arguments: `*args`

Sometimes a function should accept any number of positional arguments. Put * before the parameter name:

def add_all(*numbers):
    print(numbers)       # a tuple containing all received values
    return sum(numbers)

print(add_all(2, 4))
print(add_all(2, 4, 6, 8))

The name args is a convention, not a special word. *numbers collects extra positional arguments into a tuple. Use *args only when a variable number of values is genuinely useful; ordinary parameters are easier for beginners to read.

Variable-length keyword arguments: `**kwargs`

Sometimes a function should accept extra named options. Put ** before the parameter name:

def show_profile(**details):
    for key, value in details.items():
        print(f"{key}: {value}")

show_profile(name="Meera", city="Hyderabad", role="Student")

**details collects the named arguments into a dictionary. The name kwargs is also only a convention.

`*args` and `**kwargs` together

def report(title, *scores, **metadata):
    print(title)
    print(f"Average: {sum(scores) / len(scores):.2f}")
    print(metadata)

report("Math", 80, 90, 85, student="Ravi", term="First")

Learn the idea, but do not use these tools everywhere. Prefer clear named parameters when the number of inputs is known.

Scope

Variables created inside a function are local:

def make_message():
    message = "Done"
    return message

Prefer passing data into functions and returning results instead of changing global variables. This makes functions easier to test.

Docstrings

def area_of_circle(radius):
    """Return the area of a circle for a given radius."""
    return 3.14159 * radius ** 2

Optional extension: recursion

Recursion introduction

A recursive function calls itself and must have a base case:

def factorial(number):
    if number == 0:
        return 1
    return number * factorial(number - 1)

For many beginner problems, a loop is easier to read than recursion.

Real-world design

def collect_item():
    name = input("Item: ")
    price = float(input("Price: "))
    return name, price

def display_item(name, price):
    print(f"{name}: Rs. {price:.2f}")

Each function has one responsibility, so changing input or display does not require rewriting the calculation.

Common mistakes

Defining a function but never calling it.
Printing instead of returning a value needed elsewhere.
Using global variables unnecessarily.
Returning too early inside a loop.
Forgetting a recursion base case.
Confusing *args with a normal list parameter.
Forgetting that **kwargs collects named values in a dictionary.
Using *args or **kwargs when simple named parameters would be clearer.

Bug Hunter

Bug 1 — function never called

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

The function is defined, but nothing calls it or displays its result.

Bug 2 — printing instead of returning

def add(first, second):
    print(first + second)

total = add(2, 3)
print(total)

The function displays 5, but its returned value is None.

Bug 3 — shared mutable default

def add_task(task, tasks=[]):
    tasks.append(task)
    return tasks

The default list is created once, so later calls reuse it.

Show Bug Hunter fixes

# Bug 1
message = greet("Maya")
print(message)

# Bug 2
def add(first, second):
    return first + second

# Bug 3
def add_task(task, tasks=None):
    if tasks is None:
        tasks = []
    tasks.append(task)
    return tasks

Optional deeper look: what happens during a function call?

Python first matches arguments to parameters. It then creates a call frame, a workspace holding the function’s local names and current instruction. When the function returns, that frame finishes and the returned object goes back to the caller. Default values are created once when Python executes the def statement, which explains the shared-list bug above.

Practice

Try these problems on this page. Write the function name, parameters, return value, and one function call before writing the function body.

Problems

Write a function that returns the square of a number.
Write a function that checks whether a number is even.
Write a function that returns the largest of three values.
Write a function that calculates a bill with an optional discount.
Write a function that counts vowels in text.
Split a calculator into separate functions.
Write a function that returns all factors of a number.
Write a function that validates a password and returns a reason.
Write a recursive factorial function and test edge cases.
Design a reusable student-result module with at least four functions.

Show hints

Return the number multiplied by itself.
Return the remainder comparison.
Keep a current best value.
Give the discount a default of zero.
Loop through lowercase text.
Use one function for each arithmetic operation.
Test divisors from 1 through the number.
Return a boolean and a reason, or return a clear message.
Use a base case for zero.
Separate input, validation, calculations, and display.

Show solution ideas

def square(number): return number * number.
return number % 2 == 0.
Start with the first value and compare the other two.
Calculate the subtotal, then subtract subtotal * discount.
Count characters that occur in "aeiou".
Define add, subtract, multiply, and divide.
Return values for which number % factor == 0.
Check each password rule in order and return after the first failure.
return 1 if number == 0 else number * factorial(number - 1).
Use small functions with clear parameters and return values.

Homework

Refactor one earlier project so that input, validation, calculation, and display are handled by separate functions.

Checkpoint

Break a bill calculator into at least three functions and explain the inputs and outputs of each function.