Functions

A function packages up a piece of logic under a name, so you can run it again from anywhere in your program instead of copying and pasting the same lines every time you need them.

Defining and calling a function

def starts a function definition. Anything indented underneath it is the function's body, and it only runs when the function is called:

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

greet("Priya")
greet("Sam")

Hello, Priya!
Hello, Sam!

Returning a value

return hands a value back to whatever called the function, so it can be stored in a variable or used in a larger expression rather than just printed:

def square(n):
    return n * n

result = square(7)
print(result)
print(square(3) + square(4))

49
25

A function without an explicit return statement hands back None automatically — that's why greet() above was only ever called for its printed side effect, never assigned to a variable.

Default arguments

Giving a parameter a default value makes it optional — callers can leave it out and get the default, or supply their own to override it:

def power(base, exponent=2):
    return base ** exponent

print(power(5))
print(power(5, 3))

25
125

Keyword arguments

You can also pass arguments by name instead of by position, which makes a call self-documenting and lets you skip earlier defaults without listing them:

def describe_pet(name, animal="dog"):
    print(name + " is a " + animal)

describe_pet("Rex")
describe_pet(name="Whiskers", animal="cat")

Rex is a dog
Whiskers is a cat