Lists in Python

Lists are the equivalent of a simple array in JavaScript.

Lists have the following properties:

  • They are ordered
  • They are mutable and can be modified
  • They allow duplicate members
  • They are indexed
  • You can increase/decrease their length by adding/removing new members

Lists are denoted with [...]

Duplicating lists

When we want to duplicate a list, we can’t just reassign the list to a new variable and expect this to be a copy.

If we edit this “copy” it will update the original list since it copies the pointer and will therefore point to the same address in memory. Instead we have to use the List copy() function which returns a new list and doesn’t modify the original list.

Relatedly, we should distinguish those List methods and functions which create a new list (non-destructive) and those that mutate the existing list.

Basic usage

# Defining a list
list1 = ['John', 'Paul', 'George', 'Ringo']
list2 = [4]

# Empty list
list3 = []  # empty list
list3 = list()  # Also empty list

# Nested list
list5 = [[2, 3], [6, 8]]

Slicing

list1 = ['John', 'Paul', 'George', 'Ringo']

print(list1[1])
print(list1[-1])
print(list1[1:3])
print(list1[:3])
print(list1[1:])

"""
'John'
'Ringo'
['Paul', 'George']
['John', 'Paul', 'George']
['Paul', 'George', 'Ringo']
"""

Adding additional values to existing list

Single value

list1 = ['John', 'Paul', 'George', 'Ringo']

# Add single element to the end of a list
list1.append('Pete')
# ['John', 'Paul', 'George', 'Ringo', 'Pete']

Multiple values

list1.extend(['Albert', 'Bob'])
list1 += ['Ginger', 'Sporty']
# ['John', 'Paul', 'George', 'Ringo', 'Pete', 'Albert', 'Bob', 'Ginger', 'Sporty']

Specific index


## Insert at specific index

a_list = ['Adele', 'Madonna', 'Cher']
a_list.insert(1, 'Paloma')
print(a_list)

# ['Adele', 'Paloma', 'Madonna', 'Cher']

Removing elements

We distinguish del from remove when removing elements from lists:

  • del requires an index value
  • remove requires a value reference (i.e. the name of the element rather than

del is simple deletion whereas remove searches the list. Therefore del is more efficient.

del

my_list = [10, 20, 30, 40, 50]
del my_list[1]
print(my_list)  # Output will be [10, 30, 40, 50]

We can remove multiple items at once via a slice:

my_list = [10, 20, 30, 40, 50]

# Delete the elements from index 1 to 3 (inclusive of start index and exclusive of end index)
del my_list[1:4]

# Print the updated list
print(my_list)  # Output will be [10, 50]

remove()

my_list = [10, 20, 30, 40, 50]

# Remove the element with value 30
my_list.remove(30)

# Print the updated list
print(my_list)  # Output will be [10, 20, 40, 50]

If the value is not found in the list, remove()`` will raise a ValueError. To avoid this, you can check whether the value exists in the list before calling remove()`

pop()

Remove and return the element removed

list6 = ['Once', 'Upon', 'a', 'Time']
print(list6.pop(2))
# a

Return index of a list element

list7 = [2, 3, 6, 8]
print(list7.index(8))
# 3

list6 = ['Once', 'Upon', 'a', 'Time']
print(list6.index('a'))
# 2

Nesting lists

l1 = [1, 43.5, 'Phoebe', True]
l2 = ['apple', 'orange', 31]
root_list = ['John', l1, l2, 'Denise']
print(root_list)
# ['John', [1, 43.5, 'Phoebe', True], ['apple', 'orange', 31], 'Denise']

Merging lists

list1 = [1, 2, 3]
list2 = [4, 5, 6]

merged_list = list1 + list2
print(merged_list)  # Output: [1, 2, 3, 4, 5, 6]

Flattening a two-dimensional list (matrix)

Use list comprehension for each row in the matrix:

 matrix = [
    [9, 3, 8, 3],
    [4, 5, 2, 8],
    [6, 4, 3, 1],
    [1, 0, 4, 5],
]

flat = [item for row in matrix for item in row]
# [9, 3, 8, 3, 4, 5, 2, 8, 6, 4, 3, 1, 1, 0, 4, 5]

See also

Sorting lists in Python

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