Functions

You've been using functions all along: print(), len(), int(). A function is a named block of code you can run whenever you want. Now it's time to write your own.

Defining functions

Use def, a name, parentheses, and a colon. The indented lines below are the function's body:

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

Output:

Hello!
Hello!

Defining a function doesn't run it. The code only runs when you call it with greet(). Functions let you write something once and use it many times.

Parameters and arguments

Functions become powerful when you can give them input. The names in the parentheses of the definition are parameters. The actual values you pass when calling are arguments.

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

Output:

Hello, Ada!
Hello, Alan!

With several parameters, arguments are matched up by position:

def add(a, b):
    print(a + b)
 
add(2, 3)   # 5

Return values

print() shows something on the screen, but the program can't use it afterward. return hands a value back to the caller, so you can store it or use it in more calculations:

def square(n):
    return n * n
 
result = square(4)
print(result)            # 16
print(square(3) + 1)     # 10

The moment Python reaches a return, the function ends. Anything after it in the body is skipped.

def sign(n):
    if n < 0:
        return "negative"
    if n == 0:
        return "zero"
    return "positive"

A function can return several values at once. They come back as a tuple, which you can unpack:

def min_max(numbers):
    return min(numbers), max(numbers)
 
low, high = min_max([4, 9, 2])
print(low, high)   # 2 9

Default arguments

You can give a parameter a default value. If the caller leaves it out, the default is used:

def power(base, exponent=2):
    return base ** exponent
 
print(power(5))      # 25
print(power(2, 10))  # 1024

Parameters with defaults must come after the ones without.

Keyword arguments

You can also pass arguments by name. This makes calls easier to read, and the order no longer matters:

def describe(name, species, age=1):
    print(f"{name} is a {age}-year-old {species}.")
 
describe("Rex", "dog", 3)
describe(species="cat", name="Mia")
describe("Bo", "bird", age=2)

Output:

Rex is a 3-year-old dog.
Mia is a 1-year-old cat.
Bo is a 2-year-old bird.

When you mix the two styles, positional arguments must come first.

Variable scope

A variable created inside a function is local. It exists only while the function runs and can't be seen from outside:

def add_five(price):
    total = price + 5
    return total
 
print(add_five(10))   # 15
print(total)          # NameError: 'total' is not defined

A function can read variables from outside, but assigning to a name inside a function creates a brand-new local variable, even if the same name exists outside:

x = 10
 
def change():
    x = 99
    print(x)   # 99
 
change()
print(x)       # 10

Putting it together

def is_even(n):
    return n % 2 == 0
 
def count_evens(numbers):
    total = 0
    for n in numbers:
        if is_even(n):
            total += 1
    return total
 
print(count_evens([1, 2, 3, 4, 6]))   # 3

Small functions built from other small functions are how real programs are made.

In the puzzles for this lesson, the starter code gives you the function header and the input/output lines. Your job is to fill in the body.

That's the last lesson, so well done on finishing the course! Now try the final three puzzles.