Python Functions

Defining functions

Use the def keyword followed by the function name and parentheses.

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

greet("Alice")

Return values

Use return to send a value back from the function.

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

result = add(3, 4)
print(result)  # 7

Default parameters

Give parameters default values that can be overridden.

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

greet("Alice")
greet("Bob", "Hi")

Try it yourself

Run the example below. Edit it and see what changes.

def square(n):
    return n * n

print(square(5))
print(square(3))
 

Common mistakes

• ⚠Forgetting the colon after def and the parentheses.
• ⚠Forgetting to return a value — the function returns None by default.
• ⚠Mutable default arguments (like lists) are shared between calls.