Lists#

Lists are used to store a collection of objects but are more flexible than tuples. You can create lists using the list function with another iterable object or square brackets []:

list1 = list((1, 2, 3))
print('list1', list1)

list2 = [4, 8, 9]
print('list2', list2)

list1 [1, 2, 3]
list2 [4, 8, 9]

You can access elements of the list by indexing and slicing it:

letters = ['a', 'b', 'c', 'd', 'e']
print('Letters:', letters)
print('First character:', letters[0])
print('Second character:', letters[1])
print('Last character:', letters[-1])
print('Every second character:', letters[::2])

Letters: ['a', 'b', 'c', 'd', 'e']
First character: a
Second character: b
Last character: e
Every second character: ['a', 'c', 'e']

Unlike tuples you can alter the elements of a list after instancing it:

letters = ['a', 'b', 'c', 'd', 'e']
print(letters)

print('Changing the third character')

letters[2] = 'z'
print(letters)

['a', 'b', 'c', 'd', 'e']
Changing the third character
['a', 'b', 'z', 'd', 'e']

You can also assign new values to slices:

letters = ['a', 'b', 'c', 'd', 'e']
print(letters)

print('Changing the first three characters')
letters[:3] = ['x', 'y', 'z']
print(letters)

['a', 'b', 'c', 'd', 'e']
Changing the first three characters
['x', 'y', 'z', 'd', 'e']

Concatenating Lists#

The + operator acts on lists in a similar way to strings, concatenating the two lists:

list1 = [1, 2, 3]
list2 = ['a', 'b', 'c']

print(list1 + list2)

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

list.append()#

You can add elements to the end of the list using the .append() method:

letters = ['a', 'b', 'c', 'd', 'e']
print(letters)

print('Appending an additional letter')

letters.append('f')
print(letters)

['a', 'b', 'c', 'd', 'e']
Appending an additional letter
['a', 'b', 'c', 'd', 'e', 'f']

list.insert()#

If you want to insert an element into a specific place in the list you can use the .insert() method. This takes the index and the object you want to add as the arguments:

numbers = [1, 2, 4, 5, 6]
print(numbers)

print('Inserting number 3 at index 2')

numbers.insert(2, 3)
print(numbers)

[1, 2, 4, 5, 6]
Inserting number 3 at index 2
[1, 2, 3, 4, 5, 6]

lists.remove()#

If you want to remove the first instance of an element of a list with a specific value you can use the .remove() method:

numbers = [1, 2, 1, 3, 4]
print(numbers)

print('Removing first 1 from numbers')

numbers.remove(1)
print(numbers)

[1, 2, 1, 3, 4]
Removing first 1 from numbers
[2, 1, 3, 4]

list.pop()#

If you want to retrieve and remove an element at a particular index you can use the .remove() method, which takes the index of the element you want to retrieve as the argument:

numbers = [1, 2, 3, 4, 5]
print(numbers)

print('Retrieving number at index 2:', numbers.pop(2))

print(numbers)

[1, 2, 3, 4, 5]
Retrieving number at index 2: 3
[1, 2, 4, 5]

List Comprehension#

If you are not familiar with for loops you may wish to read the page Python Standard Library/Loops/For Loops before returning to this section.

There will be many times you will want to automate the creation of a list. You can use loops for this but can become impractical. A nice way to generate lists is using list comprehension:

#Generating a list of integers in ascending order
numbers = [i for i in range(6)]
print(numbers)

[0, 1, 2, 3, 4, 5]

You can treat the for inside the list just like a for loop, including looping through collections:

string = 'abcdefg'

#Generating a list of characters from a string
char_list = [char for char in string]
print(char_list)

['a', 'b', 'c', 'd', 'e', 'f', 'g']

Only use list comprehension if you are interested in the list itself. Do not use it in place of a for loop.

You can also nest list comprehension:

print([[i + j for j in range(3)] for i in range(4) ])

[[0, 1, 2], [1, 2, 3], [2, 3, 4], [3, 4, 5]]