3. Data Structures · lesson 5 of 17 · 1 min · January 1, 2024

Lists

Probably the most fundamental data structure in Python is the list.

Probably the most fundamental data structure in Python is the list. A list is simply an ordered collection. (It is similar to what in other languages might be called an array, but with some added functionality.)

basics.py

integer_list = [1, 2, 3]
heterogeneous_list = ["string", 0.1, True]
list_of_lists = [ integer_list, heterogeneous_list, [] ]

list_length = len(integer_list)   # equals 3
list_sum = sum(integer_list)      # equals 6

Indexing

You can get or set the nth element of a list with square brackets:

indexing.py

x = range(10)   # is the list [0, 1, ..., 9]
zero = x[0]     # equals 0, lists are 0-indexed
one = x[1]      # equals 1
nine = x[-1]    # equals 9, 'Pythonic' for last element
eight = x[-2]   # equals 8, 'Pythonic' for next-to-last element

x[0] = -1       # now x is [-1, 1, 2, 3, ..., 9]

Slicing

You can also use square brackets to "slice" lists:

slicing.py

first_three = x[:3]                 # [-1, 1, 2]
three_to_end = x[3:]                # [3, 4, ..., 9]
one_to_four = x[1:5]                # [1, 2, 3, 4]
last_three = x[-3:]                 # [7, 8, 9]
without_first_and_last = x[1:-1]    # [1, 2, ..., 8]
copy_of_x = x[:]                    # [-1, 1, 2, ..., 9]

Membership

Python has an in operator to check for list membership:

membership.py

1 in [1, 2, 3]    # True
0 in [1, 2, 3]    # False

[!WARNING] This check involves examining the elements of the list one at a time, which means that you probably shouldn’t use it unless you know your list is pretty small (or unless you don’t care how long the check takes).

Concatenation

It is easy to concatenate lists together:

extend.py

x = [1, 2, 3]
x.extend([4, 5, 6])     # x is now [1,2,3,4,5,6]

If you don’t want to modify x you can use list addition:

addition.py

x = [1, 2, 3]
y = x + [4, 5, 6]       # y is [1, 2, 3, 4, 5, 6]; x is unchanged

More frequently we will append to lists one item at a time:

append.py

x = [1, 2, 3]
x.append(0)      # x is now [1, 2, 3, 0]
y = x[-1]        # equals 0
z = len(x)       # equals 4

Unpacking

It is often convenient to unpack lists if you know how many elements they contain:

unpacking.py

x, y = [1, 2]    # now x is 1, y is 2

although you will get a ValueError if you don’t have the same numbers of elements on both sides.

It’s common to use an underscore for a value you’re going to throw away:

underscore.py

_, y = [1, 2]    # now y == 2, didn't care about the first element