Python List: The Definitive Guide

List is one of the most frequently used containers in Python that lets you store many items in one place.

How to create a python list

You can create a list by wrapping values in square brackets []:

ages = [10, 40, 20]

print(ages)

Output:

[10, 40, 20]

Here, we’ve got a list with three items: 10, 40, and 20.

Your list can have items of any type, not just numbers:

# mixed types in a single list
list_a = [1, "two", 3]

# list with repeated values
list_b = [1, 1, 2, 1]

# an empty list
list_c = []

The list constructor

The list constructor lets you make a list without the square brackets from any existing sequence:

# an empty list
empty_list = list()

# a list of characters
char_list = list("abc")

print(char_list)

Output:

['a', 'b', 'c']

Let’s create a list of numbers with the range function:

numbers = list(range(1, 10, 3))

print(numbers)

Output:

[1, 4, 7]

Getting an item from a list by its index

To grab an item from a list, just specify its number inside square brackets [<index>]:

l = ["zero", "one", "two"]
item = l[2]

print(item)

Output:

two

how to change an item in a list

To change an item in the list, simply assign a new value to it by its index using the = operator:

numbers = [1, 2, 3]
print('before:', numbers)

numbers[1] = 9
print('after:', numbers)

Output:

before: [1, 2, 3]
after:  [1, 9, 3]

How to get the length of a list

Python has a function named len to find out the length of a list. Just hand it a list, and it’ll tell you how many items are in there:

numbers = [1, 2, 3, 4]

print('len:', len(numbers))

Output:

len: 4

Negative index

Python allows us to access items from the end using a negative index:

ages = [18, 24, 31]

print('last one:   ', ages[-1])
print('second last:', ages[-2])

Output:

last one:    31
second last: 24

‘for’ loop with lists

Python lets us use the for loop with lists. Just specify the list after the in keyword:

students = ["John", "Kate", "Mike"]

for student in students:
  print(student)

Output:

John
Kate
Mike

List of lists

You can have lists within lists to whatever depth you like:

matrix = [
  [0, 1, 0],
  [0, 2, 0],
  [0, 0, 0],
]

# get an item
print("matrix[0][1] =", matrix[0][1])

# change an item
matrix[2][2] = 2
print(matrix)

Output:

matrix[0][1] = 1
[[0, 1, 0], [0, 2, 0], [0, 0, 2]]

Adding items to a list

We’ve seen how to make a list and tweak its individual items. But how do we add stuff to an existing list? The list type offers a few neat methods for this.

list.append

The .append(item) method lets you add an item at the end of a list:

l = [0, 1, 2]
l.append(3)
print(l)

Output:

[0, 1, 2, 3]

list.extend

If you wanna throw a bunch of items at the end all at once, go for the .extend(items) method:

a = [0, 1, 2]
b = [3, 4]
a.extend(b)
print(a)

Output:

[0, 1, 2, 3, 4]

list.insert

When you’re need to add an item in the middle of a list, .insert(item) is the way to go:

l = [3, 1, 0]
# shove 2 into index 1
l.insert(1, 2)
print(l)

Output:

[3, 2, 1, 0]

Removing an item from a list

Now that we’ve learned how to add items to an existing list, let’s learn how to remove them. There are a few methods provided by the list type for this, but let’s start with the special keyword del.

The del keyword

Python allows you to remove any item from a list using its index with the del keyword:

l = [0, -3, 1]
del l[1]
print(l)

Output:

[0, 1]

list.remove

The .remove(value) method removes the first occurrence of a value in the list:

l = [1, -3, 2, -3]
l.remove(-3)
print(l)

Output:

[1, 2, -3]

list.pop

The .pop(index=-1) method removes the item at the specified position and returns its value:

l = [1, -3, 2, -4]

# by default, the last item is removed
print("pop:", l.pop(), "  l:", l)

# here, we're removing the second item
print("pop:", l.pop(1), "  l:", l)

Output:

pop: -4   l: [1, -3, 2]
pop: -3   l: [1, 2]

list.clear

The .clear() method removes all items from the list:

l = [1, 2, 3]
l.clear()

print(l)

Output:

[]

Getting a slice of a list

Python offers an elegant way to fetch specific parts of a list. To get a slice, use the format [<start>:<end>:<step>].

The <end> index item won’t be included in the slice. You can leave out the start, end, and step:

l = [0, 1, 2, 3, 4]

# here the slice is the entire list
print("l[:]     =", l)

# here we're taking the 1st and 2nd items (end is exclusive)
print("l[1:3]   =", l[1:3])

# here we're taking all items starting from the first
print("l[1:]    =", l[1:])

# you can use negative indexes
# here we're grabbing all items starting from the second last
print("l[-2:]   =", l[-2:])

# here we're taking items from the first to third, skipping every other
print("l[0:3:2] =", l[0:3:2])

# here the list is reversed
print("l[::-1]  =", l[::-1])

Output:

l[:]     = [0, 1, 2, 3, 4]
l[1:3]   = [1, 2]
l[1:]    = [1, 2, 3, 4]
l[-2:]   = [3, 4]
l[0:3:2] = [0, 2]
l[::-1]  = [4, 3, 2, 1, 0]

You can also use slices to modify lists:

l = [0, 1, 2, 3, 4]
print("original: ", l)

l[1:3] = [9, 8, 7]
print("modified: ", l)

l[1:4] = []
print("deleted:  ", l)

Output:

original:  [0, 1, 2, 3, 4]
modified:  [0, 9, 8, 7, 3, 4]
deleted:   [0, 3, 4]

Slices are a very handy tool when working with lists. I’d suggest getting comfortable working with them. It’ll greatly help you down the line.

Searching for values in a list

Now, let’s look at how you can search for items in a list.

The in keyword

Using value in [1, 2, 3] will return True if value is in the list:

l = [1, 2, 3]
print("2 in l:", 2 in l)
print("5 in l:", 5 in l)

Output:

2 in l: True
5 in l: False

list.index

The .index(value) method gives you the index of the first item that matches the given value. If the item isn’t found, index will raise an error:

l = [1, 2, 3, 2]
print("l.index(2):", l.index(2))
print()

l.index(5) # <- will raise an error

Output:

l.index(2): 1

Traceback (most recent call last):
  ...
ValueError: 5 is not in list

If you only want to search within a part of the list, you can specify start and end arguments:

l = [1, 2, 3, 2, 5, 6]
# find the value 2 between the third and fifth items
print(l.index(2, 2, 4))

Output:

3

As you can see, the index method returned the position of the second instance of the number 2.

list.count

The .count(value) method can be used to count how many times a specific value appears in a list:

l = [1, 2, 3, 2, 3]
print(l.count(3))

Output:

2

sorting a list

If you need to sort a list, you can use the .sort() method:

l = [3, 2, 4, 1]
l.sort()
print(l)

Output:

[1, 2, 3, 4]

Now, if you want to sort in descending order, you can add reverse=True:

l = [3, 2, 4, 1]
l.sort(reverse=True)
print(l)

Output:

[4, 3, 2, 1]

You can also change the criteria for sorting:

l = ["one", "two", "ten"]
l.sort()
print("normal sort:", l)

# Here, we're sorting based on length
l.sort(key=len)
print("key sort:   ", l)

Output:

normal sort: ['ten', 'two', 'one']
key sort:    ['two', 'one', 'ten']

reversing a list

You already know that you can reverse a list using slices with a negative step:

l = [1, 2, 3]
print(l[::-1])

Output:

[3, 2, 1]

But in Python, you can also reverse a list without using slices. Just use the .reverse() method:

l = [1, 2, 3]
l.reverse()
print(l)

Output:

[3, 2, 1]

making a list copy

Just like with reversing, you can use slices to make a copy:

a = [1, 2, 3]
b = a[:]
a[1] = 0

print("a:", a)
print("b:", b)

Output:

a: [1, 0, 3]
b: [1, 2, 3]

But if you’re not a fan of slices, you can use the .copy() method:

a = [1, 2, 3]
b = a.copy()
a[1] = 0

print("a:", a)
print("b:", b)

Output is the same as before:

a: [1, 0, 3]
b: [1, 2, 3]

List comprehensions in Python

Python has a neat way to create lists based on certain conditions. These are called list comprehensions:

# Simple comprehension
print("1.", [x for x in range(0, 3)])

# Comprehension with filtering
print("2.", [x for x in range(0, 3) if x % 2 == 0])

# Comprehension with modification
print("3.", [x * 2 for x in range(0, 3)])

# Double loop comprehension
print("4.", [i * j for i in [1, 2] for j in range(1, 3)])

Output:

1. [0, 1, 2]
2. [0, 2]
3. [0, 2, 4]
4. [1, 2, 2, 4]

Unpacking lists

In Python, you can break down a list into individual values. To do this, just assign the list elements to variables:

a, b, c = [1, 2, 3]

print("sum:", a + b + c)

Output:

sum: 6

But, if the number of elements in the list doesn’t match the number of variables, you’ll get an error:

a, b = [1, 2, 3, 4, 5]

Error:

Traceback (most recent call last):
  ...
ValueError: too many values to unpack (expected 2)

Using the type function with lists

If you ever need to check whether a variable holds a list, you can use the type function:

number = 1
l = [1, 2, 3]

print("Is number a list?:", type(number) == list)
print("Is l a list?:     ", type(l) == list)

Output:

Is number a list?: False
Is l a list?:      True

Adding lists together

In Python, you can merge two lists using the + operator:

a = [1, 2, 3]
b = [4, 5, 6]
print(a + b)

Output:

[1, 2, 3, 4, 5, 6]

Multiplying lists

Sometimes, it’s handy to create a list where the values repeat. You can achieve this by multiplying a list by a number:

a = [1, 2]
b = a * 3
print(b)

Output:

[1, 2, 1, 2, 1, 2]

Comparing lists

If you have two lists and you want to compare them, you can use the standard ==:

a = [1, 2, 3]
b = [1, 2, 3]
c = [3, 2, 1]

print("Is a equal to b?:", a == b)
print("Is b equal to c?:", b == c)

Output:

Is a equal to b?: True
Is b equal to c?: False

Exercises

1. Creating and displaying a list:
   Write a Python program that lets the user input data until they enter the word “stop”. All entered data should be stored in a list. After entering “stop”, the program should display the entire list.
2. Manipulating the list:
   Write a program that asks the user to input 5 numbers. Store these numbers in a list and then display the highest, lowest, and the average of all the numbers in the list.
3. Operations with lists:
   Create two lists: one with strings and another with numbers. Write a program that combines these lists into one and displays the result. Try using methods like append and extend, and see how they impact the outcome.