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Lesson 6: Functions & Built-in Functions

Back to the main roadmap | Previous lesson: Exceptions

What is a Function?

A function is a reusable block of code that performs a specific task.

Instead of writing the same code multiple times, you write it once inside a function and call it whenever you need it.

Real-Life Example

Think of a coffee machine.

You don't rebuild the machine every time you want coffee.

You simply press a button, and the machine performs the same steps every time:

  • Grind the beans
  • Brew the coffee
  • Pour the coffee

Functions work the same way.

Defining a Function

Use the def keyword to create a function.

Example

def make_coffee():
    print("Grinding beans...")
    print("Brewing...")
    print("Coffee is ready!")

make_coffee()
make_coffee()

Output

Grinding beans...
Brewing...
Coffee is ready!
Grinding beans...
Brewing...
Coffee is ready!

Understanding the Syntax

def make_coffee():
  • def → Defines a function.
  • make_coffee → Function name.
  • () → Parentheses for inputs (parameters).
  • : → Starts the function body.
  • Indented lines → Code that belongs to the function.

A function does nothing until you call it.

Parameters and Arguments

Functions become much more useful when they can receive data.

Example

def greet(name):
    print("Hello, " + name + "!")

greet("Alex")
greet("Sam")

Output

Hello, Alex!
Hello, Sam!

Explanation

  • name is a parameter.
  • "Alex" and "Sam" are arguments.

Think of the parameter as an empty box that gets filled when the function is called.

Multiple Parameters

Functions can accept more than one input.

Example

def introduce(name, age):
    print(name + " is " + str(age) + " years old.")

introduce("Alex", 10)

Output

Alex is 10 years old.

Arguments are matched by their position.

Keyword Arguments

Instead of relying on position, you can specify parameter names.

Example

introduce(age=10, name="Alex")

The output is exactly the same.

Using keyword arguments makes code easier to read and allows the arguments to be written in any order.

return vs print

This is one of the most important concepts in Python.

print()

Displays information on the screen.

def add_print(a, b):
    print(a + b)

add_print(3, 5)

Output

8

The value is displayed but not returned.

return

Returns a value back to the caller.

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

result = add(3, 5)

print(result)

Output

8

The returned value can be stored in a variable or used later.

Comparing print and return

def add_print(a, b):
    print(a + b)

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

x = add_print(3, 5)
y = add_return(3, 5)

print(x)
print(y)

Output

8
None
8

Why?

  • add_print() only displays the value.
  • It doesn't return anything.
  • Python automatically returns None when there is no return statement.
  • add_return() returns the value 8.

Remember:

  • print() shows
  • return gives back

A Function Stops at return

When Python reaches a return statement, the function ends immediately.

def check_age(age):
    if age < 0:
        return "Invalid age"

    return "Valid age"

    print("This never runs.")

The last print() is never executed because the function has already finished.

Default Parameter Values

Parameters can have default values.

Example

def greet(name="friend"):
    print("Hello, " + name + "!")

greet()
greet("Alex")

Output

Hello, friend!
Hello, Alex!

If no argument is supplied, Python uses the default value.

*args

Sometimes you don't know how many arguments will be passed.

Use *args.

Example

def add_all(*numbers):
    total = 0

    for n in numbers:
        total += n

    return total

print(add_all(1, 2, 3))
print(add_all(1, 2, 3, 4, 5))

Output

6
15

*numbers collects all positional arguments into a tuple.

**kwargs

Sometimes you don't know the names of the arguments either.

Use **kwargs.

Example

def describe_person(**info):
    for key, value in info.items():
        print(key + ":", str(value))

describe_person(
    name="Alex",
    age=10,
    city="Delhi"
)

Output

name: Alex
age: 10
city: Delhi

**kwargs stores named arguments inside a dictionary.

Variable Scope

Variables created inside a function only exist inside that function.

Example

def my_function():
    x = 5
    print(x)

my_function()

print(x)

Output

5
NameError: name 'x' is not defined

x is a local variable.

It disappears after the function finishes.

Global Variables

Variables created outside functions are called global variables.

Example

count = 0

def increase():
    global count
    count += 1

increase()

print(count)

Output

1

The global keyword tells Python to use the variable outside the function.

Built-in Functions

Python includes many useful functions that are always available.

Function Purpose Example
print() Display output print("Hello")
input() Read user input input("Name: ")
len() Count items len("hello") → 5
type() Get data type type(5)
int() Convert to integer int("25")
float() Convert to float float("3.14")
str() Convert to string str(25)
max() Largest value max(2, 5, 1)
min() Smallest value min(2, 5, 1)
sum() Sum numbers sum([1,2,3])
sorted() Sort items sorted([3,1,2])
range() Generate numbers range(5)
abs() Absolute value abs(-5)

You don't need to memorize them all.

You'll naturally learn them as you continue programming.

Practice Program

Create a file named lesson6.py.

def calculate_area(length, width=1):
    area = length * width
    return area


# Basic call
print(calculate_area(5, 3))

# Using the default value
print(calculate_area(5))


def describe_student(name, **details):
    print(f"Student: {name}")

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


describe_student(
    "Alex",
    age=10,
    grade=5,
    favorite_subject="Math"
)


def safe_divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        return "Cannot divide by zero"


print(safe_divide(10, 2))
print(safe_divide(10, 0))


total = 0

def add_to_total(x):
    global total
    total += x


add_to_total(5)
add_to_total(10)

print(total)

Output

15
5
Student: Alex
  age: 10
  grade: 5
  favorite_subject: Math
5.0
Cannot divide by zero
15

Key Takeaways

  • Functions let you reuse code.
  • Use def to create a function.
  • Call a function using its name followed by ().
  • Parameters receive values from arguments.
  • return sends a value back.
  • print() displays output but does not return it.
  • A function ends immediately when it reaches return.
  • Default parameters provide fallback values.
  • *args accepts multiple positional arguments.
  • **kwargs accepts multiple named arguments.
  • Local variables exist only inside functions.
  • Global variables exist outside functions and can be modified using global.
  • Python provides many built-in functions like print(), len(), sum(), and sorted().

Back to the main roadmap | Previous lesson: Exceptions