Functions help extract out common code blocks.

Let's define a function print_greeting().

In [1]: 
def print_greeting():
    print("Hi there, how are you?")
    print("Long time no see.")

And call it:

In [2]: 
print_greeting()

Hi there, how are you?
Long time no see.

That's a bit impersonal.

In [3]: 
def print_greeting(name):
    print("Hi there, {0}, how are you?".format(name))
    print("Long time no see.")
In [4]: 
print_greeting("Andreas")

Hi there, Andreas, how are you?
Long time no see.

But we might not know their name.

(And we just changed the interface of print_greeting!)

In [6]: 
def print_greeting(name="my friend"):
    print("Hi there, {0}, how are you?".format(name))
    print("Long time no see.")
In [7]: 
print_greeting("Andreas")
print_greeting()

Hi there, Andreas, how are you?
Long time no see.
Hi there, my friend, how are you?
Long time no see.

Function parameters work like variables.

So what does this do?

In [8]: 
def my_func(my_list):
    my_list.append(5)
    
l = [1,2,3]
my_func(l)
print(l)

[1, 2, 3, 5]

Can be very surprising!

Define a better function my_func_2:

In [9]: 
def my_func_2(my_list):
    return my_list + [5]
In [10]: 
l = [1,2,3]
l2 = my_func_2(l)
print(l)
print(l2)

[1, 2, 3]
[1, 2, 3, 5]