If you've done programming before, you may have come across arrays. An array is variable that can store a defined amount of date and if you remember, a variable is simply a defined memory location to holds a single value.

A list is Python's answer to the traditional array. The differece is that traditionally, arrays must be defined with a predefined size however, lists can be defined as empty and items can be added or removed at will.

Each location of data stored in a list is referenced through an index which represents the position of the value in the list. Indexes start from 0.

Typical layout

Looking at the above, each "block" represents a location to store data. In our case we're storing 6 items.

If we wanted to get the third item in the list, we'd reference index 2: my_list[2]

If we wanted the item in first position: mylist[0]

For lists we follow the same principle of variables in terms of naming conventions, but we include the square bracket [] and values are separated by ,.

my_list = [] # empty list

print(my_list)

new_list = [1,2,3] # list with values
print(new_list)

Lists, like arrays, uses indexes to access the data stored in it. Let's look at basic example:

ports = [80,135,137,139,443,1099]

# Print the first port
print(ports[0])

# Print the third port
print(ports[2])

Getting the number of items

We use the len() function:

ports = [80,135,137,139,443]

print(len(ports))

Combining lists

To combine to a list we can do list concatenation, similar to what we do with strings.

ports = [80,443,21]

new_ports = [23,25,53]

ports = ports + new_port

print(ports)

Working with lists in a for loop

ports = [80,135,137,139,443]

# index syntax
for x in range(len(ports)):
    print(f"{x}: {ports[x]}")

# Non-index syntax
for port in ports:
    print(f"port: {port}")

Changing items in a list

ports = [80,135,137,139,443]

print(ports)

# Changing 137 to 53
ports[2] = 53

print(ports)

Taking input and storing in a list

targets = []

while True:
    target = input("Enter target(type quit to exit): ")
    if target != "quit":
        targets.append(target)
    else:
        break

print(targets)

We've mentioned the index is used to access the data in the list. Here are a few things we can do with indexes:

usernames = ["admin","test_user","sqladmin","root","www-data","webmin",]

# Print the first item
print(usernames[0])

# Print the third item
print(usernames[2])

# Print the first three items
print(usernames[0:2]) # option 1
print(usernames[:2]) # option 2

# Print the last item
print(usernames[-1])

# Print the last two items
print(usernames[-2:])

# Print every other item
print(usernames[::2]) # ascending order
print(usernames[::-2]) # descending order

There are a few methods that come along with lists that we can use to remove items. These are:

1. pop()
2. remove()
3. del()

You may be wondering why can we just assign it nothing similar to this usernames[2] = "". The thing is "" is an empty string, therefore still has a value. The length is 0 however.

So what are the differences between the mentioned methods?

pop()

The pop(x) method removes the item at index 'x' and returns it. Think of it popping from the list and putting it aside.

For example:

my_list = ["mon","tues","wed","thur","fri","fridady","sat","sun"]

print(my_list.pop(5))
print(my_list)

remove()

The remove(item) method, removes the first occurence of an 'item' from the list.

# continuing with the my_list from the previous example

print(my_list.remove("mon"))

del()

del(x) removes the specified index 'x'.

del my_list[2]
print(my_list)

Python makes it easy to slice lists which allows you to work on a specific section of a list.

The operator for this is :

With this operator, you can specify where to start and end a slice and how to step through.

We saw this in action in the working with indexes section.

Syntax: my_list = [start:stop:step]

usernames = ["admin","test_user","sqladmin","root","www-data","webmin",]

print(usernames[1:4:2]

Let's breakdown theprint statement:

We'll print from the usernames list, every 2nd index starting from index_1_, stopping at index 4 but not including it.

Your result should be ['test user','root']

Let's say, for example, we want every 2nd item in the list usernames[::2]

This will return every other username from the usernames list.

How about printing the last item first the decending: usernames[::-1]

Getting the last time: usernames[-1::]