We can create our own functions using this pattern

def name():
	code

def is an instruction that means "define" ( i.e. create ) a new function with the following name and code. Then the code that is underneath the def name() is indented. The indented code will be run by the computer everytime the function is called.

Example

The function itself will print "hello" and then print "world". Since the function is called three times, it will print those words three times. The function is defined once, but everytime it is called as a function instruction, the code inside the definition is run. You can also see that the print() instruction is not aligned with the def instruction, it is indented. That means there is two spaces or a tab character inserted before the code starts.

Here is an example of another function, one that has input. It takes a name variable as input and says hello.

You might ask yourself, why do we want functions? why not just write the code we need?

This is just example code, so instead imagine a task with many small parts. Like, making dinner. The small parts are something like:

prepare vegetables
boil water
heat vegetables
add spagetti to water
add sauce
wait

If you had a robot to make you dinner, it would be annoying to have to specify the whole operation in precise details everytime. It would be much nicer just to wrap up the details and call cookDinner() and the process for making dinner is defined already.

def cookDinner():
	prepare vegetables
	boil water
	heat vegetables
	add spagetti to water
	add sauce
	wait

Wrapping up details or complexities in a simple function is an example of abstraction.

Abstraction makes it easier for us to reason about the world.

Functions can call other functions, so in our cooking example, each instruction could be a function like this:

def cookDinner():
	prepareVegetables()
	boilWater()
	...

def boilWater():
	addWaterToKettle()
	turnKettleOn()
	...

Composing functions like this helps us structure the code. If we don't structure our code and we have lots of code, it can be hard to read and understand.