Functions

Defining and calling

def, a name, brackets, a colon, and an indented body:

def greet():
    print("Hello")

greet()
greet()

Hello
Hello

Defining a function does not run it. The body runs only when you call it, which is what the brackets do.

def greet():
    print("Hello")

print("defined, not called")
greet
greet()

defined, not called
Hello

The bare greet on its own line is the function object itself. It is a value like any other, and evaluating it does nothing.

Parameters and arguments

A parameter is the name in the definition. An argument is the value you pass in:

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

greet("Ada")
greet("Grace")

Hello, Ada
Hello, Grace

Pass the wrong number and Python says so rather than guessing:

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

try:
    greet()
except TypeError as problem:
    print("TypeError:", problem)

TypeError: greet() missing 1 required positional argument: 'name'

Returning a value

print shows something to a person. return hands a value back to the code that called the function. They are not interchangeable:

def double(n):
    return n * 2

result = double(5)
print(result)
print(double(result))

10
20

A function with no return gives back None:

def shout(word):
    print(word.upper())

value = shout("hey")
print(value)

HEY
None

Why bother

Three reasons, in order of how much they matter:

• A name. calculate_vat(total) explains itself; six lines of arithmetic do not.
• One place to change. Fix a bug once rather than in nine copies.
• Testable. A function that takes input and returns output can be checked.

def vat(amount, rate=0.2):
    return round(amount * rate, 2)

print(vat(100))
print(vat(100, 0.05))

20.0
5.0

Docstrings

A string on the first line of the body documents the function, and help() and your editor both read it:

def area(width, height):
    """Return the area of a rectangle."""
    return width * height

print(area.__doc__)

Return the area of a rectangle.

Functions are values

You can pass a function to another function, which is what sorted(key=...) has been doing all along:

def shout(word):
    return word.upper()

def apply_twice(fn, value):
    return fn(fn(value))

print(apply_twice(shout, "hey"))
print(sorted(["bb", "a", "ccc"], key=len))

HEY
['a', 'bb', 'ccc']

Note there are no brackets after shout when passing it. Adding them would call it and pass the result instead.