Mastering Python Lists As A Beginner

What Are Lists In Python? #

Lists (or arrays as some other languages refer to them) are one of the most commonly used data structures, and for good reason.

In a nutshell lists are an ordered, mutable collection of items.

An object is mutable if it can be changed after it has been created.

What makes lists really great is that each item in the list can store any data structure:

• In Discord you have a list of users.
• In YouTube you have a list of videos.
• If you're playing a video game, you'll often have an inventory list.
• You can even create a list of lists, where each item in the main list is actually another nested list.

How Do I Create A List In Python? #

The most common way to create a list is by using square brackets inside of which are your elements separated by commas:

passengers = ["Darth Vader", "Spock", "Josephus Miller"]

You can also create an empty list which will be populated later like so:

passengers = []

# or this:
passengers = list()

How Do Indexes Work In Python Lists? #

Generally speaking in programming, we identify items in a sequence by their index. This is a little bit different to how we would usually locate an item by its position.

parts = ["cabin", "wing", "flight deck", "payload door"]

In every day conversation we would say flight deck is the 3rd item. But in programming we would say flight deck is at index 2.

In Python and programming in general, we start counting from zero. This applies to characters in a string, items in a list and just about everything else in the world of programming.

# index:    0       1           2             3                 4
parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]

Referring To Items By Their Index #

If we know the index of an item in a list, we can access that item by referring to the index like so:

print(f"{parts[1]} is at index 1 in the parts list")

# output:
# wing is at index 1 in the parts list

Negative Indexes #

Items in a list can also be referred to by a negative index where we start at -1 for the last item:

# index:    -5       -4           -3             -2                 -1
parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]

Slicing List Items #

Accessing a single item in a list is all well and good, but what if we need to get a hold of several items. This is where slicing comes in.

The syntax for slicing in Python works like this:

my_list[start:stop:step]

• start: the index to start at (inclusive)
• stop: the index to end at (exclusive)
• step: allows you to skip (or step over) items

print(parts[1:3])
print(parts[0:4:2])

# output
# ["wing", "flight deck"]
# ["cabin", "flight deck"]

Updating A List Item #

Now that indexes make sense, we can use them to update items in our list.

parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]
parts[2] = "cargo bay"
print(parts)

# output:
# ["cabin", "wing", "cargo bay", "payload door", "rocket booster"]

List Length #

Getting the length of a list can be done by calling len() and passing in the list:

parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]
print(len(parts))

# output:
# 5

Keep in mind that the length of a list and the index of the last item are not the same. Here we can see the the length is 5 but the index of the last item is 4.

Pro Tip: The length of a list will always have a value 1 higher than the index of the last item.

Concatenating Lists #

Joining (ie: concatenate) two lists together is as simple as:

parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]
new_parts = ["airlock", "cargo bay"]

parts = parts + new_parts
# or you can do this short hand version
# parts += new_parts

print(parts)

# output:
# ["cabin", "wing", "flight deck", "payload door", "rocket booster", "airlock", "cargo bay"]

List Methods #

Lists have several built in methods available. Here are a few of he most commonly used ones.

append() #

To an item to the end of a list, use the append() method:

parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]
parts.append("airlock")
print(parts)

# output:
# ["cabin", "wing", "flight deck", "payload door", "rocket booster", "airlock"]

pop() #

You can almost think of pop() as the inverse of append(). pop() will remove and return an item from the end of a list.

parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]
popped_part = parts.pop()
print(popped_part)
print(parts)

# output:
# rocket booster
# ["cabin", "wing", "flight deck", "payload door"]

But that's not all pop() can do. Remember you learnt about list indexes?

Well you can pass in a list index to pop() which allows you to remove an item at a specific index:

parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]
popped_part = parts.pop(2)
print(popped_part)
print(parts)

# output:
# flight deck
# ["cabin", "wing", "payload door", "rocket booster"]

insert() #

If you need to insert a value into a specific position in a list, insert() is your go to:

insert(index, object)

This will insert airlock before index 2:

parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]
parts.insert(2, "airlock")
print(parts)

# output:
# ["cabin", "wing", "airlock", "flight deck", "payload door", "rocket booster"]

remove() #

The remove() list method will remove the first occurrence of a value:

parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]
parts.remove("wing")
print(parts)

# output:
# ["cabin", "flight deck", "payload door", "rocket booster"]

Alternatively if you know the index of the item which you want to remove from a list, you can use del like so:

parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]
del parts[1]
print(parts)
# ["cabin", "flight deck", "payload door", "rocket booster"]

# We can also delete a range by specifying a slice
parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]
del parts[1:3]
print(parts)
# ["cabin", "payload door", "rocket booster"]

reverse() #

As the name implies, reverse() will reverse the order of the items in your list.

parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]
parts.reverse()
print(parts)

# output:
# ['rocket booster', 'payload door', 'flight deck', 'wing', 'cabin']

Keep in mind, reverse() is an in place operation.

In place operation: Instead of returning a new copy of the object, the existing object is updated.

How Do I Check If An Item Exists In A Python List? #

Python provides an incredibly clean, intuitive way of checking whether or not an item exists in a list.

parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]
print("flight deck" in parts)

# output:
# True

We use the in keyword to check if an item is in a list. Easy peasy.

We can also check if an item isn't in a list by using not in like so:

parts = ["cabin", "wing", "flight deck", "payload door", "rocket booster"]
print("boat" not in parts)

# output:
# True

Conclusion #

It's seldom that a day will go by in your career as a Python programmer where you won't use a list in some form or the other.

Python lists are an intuitive and flexible data structure to add to your tool belt.