Defining and Calling Functions

15 min · Beginner

Functions are reusable blocks of code that perform a specific task. They prevent repetition (DRY: Don't Repeat Yourself) and make software modular and testable.

Defining and calling a function

Function is a named block of code that does one job. You write it once with def, then run it as many times as you like by calling its name.

Use the def keyword followed by the function name, parentheses (), and a colon. Indent the function body by 4 spaces.

Python

def greet(user_name):    """Display a personalized welcome greeting."""    print(f"Hello, {user_name}!")
greet("Rohan")

Example 1 · One job, many calls

Python

def show_total(price, quantity):    print(f"Total: {price * quantity}")
show_total(250, 2)show_total(99, 5)

Output

Total: 500
Total: 495

What this code does

def writes the steps once. Each call runs them again with new values.
price and quantity hold 250 and 2 on the first call, then 99 and 5 on the second.

The triple-quoted string immediately below the def line is the docstring, accessible via help(greet) or IDE tooltips.

Returning values with return

Return value is the result a function sends back to the code that called it, using return. The call then stands for that value.

Use return to send a calculated result back to the caller. Executing return terminates the function immediately.

Python

def calculate_area(length, width):    return length * width
area = calculate_area(12, 5)print(f"Area: {area}")

Example 2 · Even or odd check

Python

1def is_even(number):2    """Return True if number is even, False otherwise."""3    return number % 2 == 04
5print(is_even(10))6print(is_even(7))

Output

True
False

What this code does

is_even(10) evaluates 10 % 2 == 0, returning True.
is_even(7) evaluates 7 % 2 == 0, returning False.

Example 3 · Result goes back to the caller

Python

def add_tax(price):    return price + price * 18 // 100
total = add_tax(200) + add_tax(100)print(total)

Output

What this code does

add_tax(200) returns 236 and add_tax(100) returns 118.
Because the function returns values, the caller can add them: 236 + 118 = 354.

The function returns 60, which is assigned to the variable area.

Functions without return yield None

None is the special value Python uses for "no value". A function that ends without a return statement gives back None.

If a function finishes without encountering a return statement, Python automatically returns None.

Python

def log_action(msg):    print(f"[LOG] {msg}")
result = log_action("Task complete")print(f"Return value: {result}")

Example 4 · The value of a call with no return

Python

1def log_order(order_id):2    print(f"Order {order_id} saved")3
4result = log_order(101)5print(result)6print(result is None)

Output

Order 101 saved
None
True

What this code does

The function prints one line but sends nothing back.
So result is None, which result is None confirms.

Even though log_action printed text to the screen, its return value is None.

Next step

Check your understanding

9 quick questions

Defining and Calling Functions

15 min · Beginner

Need help? Ask a tutor

Functions are reusable blocks of code that perform a specific task. They prevent repetition (DRY: Don't Repeat Yourself) and make software modular and testable.

Defining and calling a function

Function is a named block of code that does one job. You write it once with def, then run it as many times as you like by calling its name.

Use the def keyword followed by the function name, parentheses (), and a colon. Indent the function body by 4 spaces.

Python

def greet(user_name):    """Display a personalized welcome greeting."""    print(f"Hello, {user_name}!")
greet("Rohan")

Example 1 · One job, many calls

Python

def show_total(price, quantity):    print(f"Total: {price * quantity}")
show_total(250, 2)show_total(99, 5)

Output

Total: 500
Total: 495

What this code does

def writes the steps once. Each call runs them again with new values.
price and quantity hold 250 and 2 on the first call, then 99 and 5 on the second.

The triple-quoted string immediately below the def line is the docstring, accessible via help(greet) or IDE tooltips.

Returning values with return

Return value is the result a function sends back to the code that called it, using return. The call then stands for that value.

Use return to send a calculated result back to the caller. Executing return terminates the function immediately.

Python

def calculate_area(length, width):    return length * width
area = calculate_area(12, 5)print(f"Area: {area}")

Example 2 · Even or odd check

Python

1def is_even(number):2    """Return True if number is even, False otherwise."""3    return number % 2 == 04
5print(is_even(10))6print(is_even(7))

Output

True
False

What this code does

is_even(10) evaluates 10 % 2 == 0, returning True.
is_even(7) evaluates 7 % 2 == 0, returning False.

Example 3 · Result goes back to the caller

Python

def add_tax(price):    return price + price * 18 // 100
total = add_tax(200) + add_tax(100)print(total)

Output

What this code does

add_tax(200) returns 236 and add_tax(100) returns 118.
Because the function returns values, the caller can add them: 236 + 118 = 354.

The function returns 60, which is assigned to the variable area.

Functions without return yield None

None is the special value Python uses for "no value". A function that ends without a return statement gives back None.

If a function finishes without encountering a return statement, Python automatically returns None.

Python

def log_action(msg):    print(f"[LOG] {msg}")
result = log_action("Task complete")print(f"Return value: {result}")

Example 4 · The value of a call with no return

Python

1def log_order(order_id):2    print(f"Order {order_id} saved")3
4result = log_order(101)5print(result)6print(result is None)

Output

Order 101 saved
None
True

What this code does

The function prints one line but sends nothing back.
So result is None, which result is None confirms.

Even though log_action printed text to the screen, its return value is None.

Next step

Check your understanding

9 quick questions