Functions

A function is a named block of code you can run again and again. Functions keep programs tidy and avoid repeating yourself.

Defining a function

Use the def keyword, a name, round brackets, and a colon. The body is indented:

def greet():
    print('Hello!')

greet()   # calls the function

Defining a function does not run it; you must call it with its name and brackets.

Parameters and arguments

A parameter is a placeholder in the definition; an argument is the real value you pass in:

def greet(name):
    print('Hi', name)

greet('Sara')   # Hi Sara

Returning a value

Use return to send a result back to the caller. That result can be stored in a variable:

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

total = add(2, 3)   # total is 5

A function without a return statement gives back None.

Default values

A parameter can have a default used when no argument is given:

def greet(name='friend'):
    print('Hi', name)

greet()          # Hi friend
greet('Ali')     # Hi Ali

Several parameters

A function can take more than one parameter. The arguments line up in the same order:

def area(width, height):
    return width * height

print(area(4, 5))   # 20

Using the returned value

Because return hands a value back, you can use a function inside a larger expression:

def double(n):
    return n * 2

print(double(3) + double(4))   # 14

Local variables

Variables created inside a function live only there. They cannot be seen from outside, which keeps functions self-contained:

def make():
    message = 'hi'
    return message

print(make())   # hi

Trying to use message outside the function would cause an error. This separation is a good thing: each function manages its own work.

Docstrings and good names

Give a function a clear name that says what it does, such as calculate_total. You may also add a short description in quotes on the first line, called a docstring, to remind readers of its purpose:

def square(n):
    'Return n multiplied by itself'
    return n * n

Well-named functions with a single job make a program much easier to read and fix later.