Python Lists

Lists are one of the 4 data types in Python used to store collections of data.

# List: ordered collection of items enclosed in square brackets
['John', 'Peter', 'Debora', 'Charles']

Getting values with indexes

# Access list elements using index (0-based, first element is index 0)
furniture = ['table', 'chair', 'rack', 'shelf']

furniture[0]  # Returns first element: 'table'

'table'

furniture[1]

'chair'

furniture[2]

'rack'

furniture[3]

'shelf'

Negative indexes

# Negative index: access elements from the end of the list
furniture = ['table', 'chair', 'rack', 'shelf']

furniture[-1]  # Returns last element: 'shelf'

'shelf'

furniture[-3]

'chair'

f'The {furniture[-1]} is bigger than the {furniture[-3]}'

'The shelf is bigger than the chair'

Getting sublists with Slices

# Slicing: get sublist using [start:end] syntax (end is exclusive)
furniture = ['table', 'chair', 'rack', 'shelf']

furniture[0:4]  # Returns elements from index 0 to 3 (4 excluded)

['table', 'chair', 'rack', 'shelf']

furniture[1:3]

['chair', 'rack']

furniture[0:-1]

['table', 'chair', 'rack']

# Slice from beginning: omit start index (defaults to 0)
furniture[:2]  # Returns first two elements

['table', 'chair']

# Slice to end: omit end index (defaults to end of list)
furniture[1:]  # Returns all elements from index 1 to end

['chair', 'rack', 'shelf']

furniture[:]

['table', 'chair', 'rack', 'shelf']

Slicing the complete list will perform a copy:

# Slicing creates a copy: [:] creates a shallow copy of the list
spam = ['cat', 'bat', 'rat', 'elephant']
spam2 = spam[:]  # Create a copy, not a reference
spam2

['cat', 'bat', 'rat', 'elephant']

spam.append('dog')
spam

['cat', 'bat', 'rat', 'elephant', 'dog']

spam2

['cat', 'bat', 'rat', 'elephant']

Getting a list length with len()

# len() returns the number of items in a list
furniture = ['table', 'chair', 'rack', 'shelf']
len(furniture)  # Returns 4

4

Changing values with indexes

# Modify list elements by assigning new values to indexes
furniture = ['table', 'chair', 'rack', 'shelf']

furniture[0] = 'desk'  # Replace first element
furniture

['desk', 'chair', 'rack', 'shelf']

furniture[2] = furniture[1]
furniture

['desk', 'chair', 'chair', 'shelf']

furniture[-1] = 'bed'
furniture

['desk', 'chair', 'chair', 'bed']

Concatenation and Replication

# List concatenation: combine two lists using + operator
[1, 2, 3] + ['A', 'B', 'C']  # Returns [1, 2, 3, 'A', 'B', 'C']

[1, 2, 3, 'A', 'B', 'C']

# List replication: repeat list multiple times using * operator
['X', 'Y', 'Z'] * 3  # Returns ['X', 'Y', 'Z', 'X', 'Y', 'Z', 'X', 'Y', 'Z']

['X', 'Y', 'Z', 'X', 'Y', 'Z', 'X', 'Y', 'Z']

my_list = [1, 2, 3]
my_list = my_list + ['A', 'B', 'C']
my_list

[1, 2, 3, 'A', 'B', 'C']

Using for loops with Lists

# Iterate over list elements using for loop
furniture = ['table', 'chair', 'rack', 'shelf']

for item in furniture:  # Loop through each item
    print(item)

table
chair
rack
shelf

Getting the index in a loop with enumerate()

# enumerate() returns both index and value in a loop
furniture = ['table', 'chair', 'rack', 'shelf']

for index, item in enumerate(furniture):  # Get index and item together
    print(f'index: {index} - item: {item}')

index: 0 - item: table
index: 1 - item: chair
index: 2 - item: rack
index: 3 - item: shelf

Loop in Multiple Lists with zip()

# zip() combines multiple lists element-wise in a loop
furniture = ['table', 'chair', 'rack', 'shelf']
price = [100, 50, 80, 40]

for item, amount in zip(furniture, price):  # Pair elements from both lists
    print(f'The {item} costs ${amount}')

The table costs $100
The chair costs $50
The rack costs $80
The shelf costs $40

The in and not in operators

# in operator: check if an item exists in a list
'rack' in ['table', 'chair', 'rack', 'shelf']  # Returns True

True

'bed' in ['table', 'chair', 'rack', 'shelf']

False

furniture = ['table', 'chair', 'rack', 'shelf']
'bed' not in furniture

True

'rack' not in furniture

False

The Multiple Assignment Trick

The multiple assignment trick is a shortcut that lets you assign multiple variables with the values in a list in one line of code. So instead of doing this:

furniture = ['table', 'chair', 'rack', 'shelf']
table = furniture[0]
chair = furniture[1]
rack = furniture[2]
shelf = furniture[3]

You could type this line of code:

furniture = ['table', 'chair', 'rack', 'shelf']
table, chair, rack, shelf = furniture

table

'table'

chair

'chair'

rack

'rack'

shelf

'shelf'

The multiple assignment trick can also be used to swap the values in two variables:

a, b = 'table', 'chair'
a, b = b, a
print(a)

chair

print(b)

table

The index Method

The index method allows you to find the index of a value by passing its name:

furniture = ['table', 'chair', 'rack', 'shelf']
furniture.index('chair')

1

Adding Values

append()

append adds an element to the end of a list:

furniture = ['table', 'chair', 'rack', 'shelf']
furniture.append('bed')
furniture

['table', 'chair', 'rack', 'shelf', 'bed']

insert()

insert adds an element to a list at a given position:

furniture = ['table', 'chair', 'rack', 'shelf']
furniture.insert(1, 'bed')
furniture

['table', 'bed', 'chair', 'rack', 'shelf']

Removing Values

del

del removes an item using the index:

furniture = ['table', 'chair', 'rack', 'shelf']
del furniture[2]
furniture

['table', 'chair', 'shelf']

del furniture[2]
furniture

['table', 'chair']

remove()

remove removes an item with using actual value of it:

furniture = ['table', 'chair', 'rack', 'shelf']
furniture.remove('chair')
furniture

['table', 'rack', 'shelf']

pop()

By default, pop will remove and return the last item of the list. You can also pass the index of the element as an optional parameter:

animals = ['cat', 'bat', 'rat', 'elephant']

animals.pop()

'elephant'

animals

['cat', 'bat', 'rat']

animals.pop(0)

'cat'

animals

['bat', 'rat']

Sorting values with sort()

numbers = [2, 5, 3.14, 1, -7]
numbers.sort()
numbers

[-7, 1, 2, 3.14, 5]

furniture = ['table', 'chair', 'rack', 'shelf']
furniture.sort()
furniture

['chair', 'rack', 'shelf', 'table']

You can also pass True for the reverse keyword argument to have sort() sort the values in reverse order:

furniture.sort(reverse=True)
furniture

['table', 'shelf', 'rack', 'chair']

If you need to sort the values in regular alphabetical order, pass str.lower for the key keyword argument in the sort() method call:

letters = ['a', 'z', 'A', 'Z']
letters.sort(key=str.lower)
letters

['a', 'A', 'z', 'Z']

You can use the built-in function sorted to return a new list:

furniture = ['table', 'chair', 'rack', 'shelf']
sorted(furniture)

['chair', 'rack', 'shelf', 'table']

The Tuple data type

furniture = ('table', 'chair', 'rack', 'shelf')

furniture[0]

'table'

furniture[1:3]

('chair', 'rack')

len(furniture)

4

The main way that tuples are different from lists is that tuples, like strings, are immutable.

Converting between list() and tuple()

tuple(['cat', 'dog', 5])

('cat', 'dog', 5)

list(('cat', 'dog', 5))

['cat', 'dog', 5]

list('hello')

['h', 'e', 'l', 'l', 'o']

Relevant links

• Python Data Types: A Visual Guide for Beginners
• Python Comprehensions Step-by-Step
• Python Comprehensions
• The itertools Module
• list()
• tuple()
• len()