Functions

So far every program runs once, top to bottom. A function is a set of instructions with a name, so you can run them whenever you like, with different values each time.

def double(x):
    return x * 2

print(double(5))
print(double(21))

Three things to notice:

def double(x): starts the function. double is its name, x is its parameter: a name for whatever value is passed in.
The line return x * 2 is pushed in from the left. That is how Python knows it belongs to the function. The next section explains this properly.
return x * 2 hands the result back to whoever called the function.

double(5) is a call. Python sets x to 5, runs the body, and the call is replaced by the returned value, 10, which print then shows.

Blocks and indentation

Some lines in Python own other lines. def double(x): owns the lines that make up the function. Python needs to know exactly which lines those are, and it uses the space at the start of each line to decide. This is called indentation, and it follows three rules:

A line that owns other lines ends with a colon :.
The lines it owns start with four spaces more than it does. In the editor, pressing Tab inserts four spaces for you.
The group ends at the first line that goes back to the earlier level.

The group of owned lines is called a block. One more thing before the example: writing def only stores the function. Its block runs later, each time the function is called, not at the point where it is written. Watch the difference here:

def greet():
    print("Hello")
    print("Welcome to the shop")
print("This line is not part of greet")

greet()

The output is three lines, in this order: This line is not part of greet, then Hello, then Welcome to the shop. The unindented line is outside the block, so it runs first, on its own. The two indented prints are the block, and they run only when greet() is called on the last line.

Getting the spaces wrong is an error. Run this and read the message:

def greet():
print("Hello")

IndentationError: expected an indented block means Python saw the colon and then found no indented lines. Mixing different amounts of spaces inside one block gives a similar IndentationError with a different message. Stick to four, every time.

You will meet blocks again with if and loops. The rules are identical: colon, four spaces, back out to end the block.

return is not print

This trips up everyone at first. print shows a value on the screen. return gives a value back to the code that called the function. A function that prints but does not return gives back nothing:

def double_and_print(x):
    print(x * 2)

result = double_and_print(5)
print("result is", result)

The first line of output is 10, from the print inside. The second says result is None, because nothing was returned. None is Python's word for "no value".

The exercises on this site check what your function returns. If a check says it got None, you probably printed instead of returning.

Several parameters

Separate parameters with commas. The values are matched up in order.

def line_total(price, quantity):
    return price * quantity

print(line_total(40, 3))
print(line_total(2.5, 4))

Functions can use other functions

Once a function exists, any later code can call it, including other functions.

def line_total(price, quantity):
    return price * quantity

def total_with_tax(price, quantity):
    subtotal = line_total(price, quantity)
    return subtotal * 1.1

print(total_with_tax(40, 3))

subtotal here is a name that exists only inside total_with_tax. Names created inside a function disappear when the function returns.

Try it

Triple

Write a function triple(x) that returns three times x.

triple(5)   ->  15
triple(2.5) ->  7.5

def triple(x):
    ...

Change due

Write a function change_due(paid, cost) that returns how much change a customer should get back.

change_due(50, 35)  ->  15
change_due(20, 20)  ->  0

def change_due(paid, cost):
    ...

Celsius to Fahrenheit

The shop's freezer shows temperatures in Celsius, but the supplier's paperwork uses Fahrenheit. Write to_fahrenheit(c) that converts. The formula is c * 9 / 5 + 32.

to_fahrenheit(0)    ->  32.0
to_fahrenheit(100)  ->  212.0
to_fahrenheit(10)   ->  50.0

Some inputs give long decimals, such as to_fahrenheit(-18) printing -0.3999999999999986 rather than -0.4. That is how computers store decimal numbers, and the checks allow for it.

def to_fahrenheit(c):
    ...

Next: True and False.