Python Lists

A list stores many values in one variable, in order. You will use lists in almost every Python program you write, from a shopping cart to a set of test results.

What is a list in Python?

A list is written with square brackets, with items separated by commas. One list can hold numbers, strings, booleans, None and even other lists.

fruits = ["apple", "banana", "cherry"]mixed = [1, "two", 3.0, True, None]empty = [] print(fruits)print(type(fruits))print(len(mixed), len(empty))

['apple', 'banana', 'cherry']
<class 'list'>
5 0

You can also build a list from any iterable with list(), for example from a string or a range.

print(list("abc"))print(list(range(1, 6)))

['a', 'b', 'c']
[1, 2, 3, 4, 5]

Indexing and slicing

Every item has a position, called its index. Counting starts at 0 from the left and at -1 from the right.

nums = [10, 20, 30, 40, 50] print(nums[0])      # first itemprint(nums[-1])     # last itemprint(nums[1:4])    # items at index 1, 2, 3print(nums[::-1])   # reversed copy

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

Reading an index that does not exist raises an IndexError. A slice never does: it just returns fewer items.

nums = [10, 20, 30]print(nums[1:10])print(nums[5])

[20, 30]
Traceback (most recent call last):
  File "main.py", line 3, in <module>
    print(nums[5])
          ~~~~^^^
IndexError: list index out of range

Add, change and remove items

cart = ["pen", "book"] cart.append("bag")        # add to the endcart.insert(1, "ink")     # add at index 1cart[0] = "pencil"        # replace an itemprint(cart) last = cart.pop()         # remove and return the last itemprint(last, cart) cart.remove("ink")        # remove by valueprint(cart)

['pencil', 'ink', 'book', 'bag']
bag ['pencil', 'ink', 'book']
['pencil', 'book']

Python list methods you will use most

append() adds its argument as one item, while extend() adds each item separately:

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

[1, 2, [3, 4]]
[1, 2, 3, 4]

Loop through a list

A for loop visits each item in order. enumerate() gives you the position and the item together, so you do not need a counter variable.

tasks = ["write tests", "run tests", "fix bugs"] for number, task in enumerate(tasks, start=1):    print(f"{number}. {task}")

1. write tests
2. run tests
3. fix bugs

Use in to check whether a value is in a list:

browsers = ["chromium", "firefox", "webkit"]print("firefox" in browsers)print("edge" in browsers)

True
False

Sort a list

scores = [72, 95, 58, 88] print(sorted(scores))          # new sorted listprint(scores)                  # original is unchanged scores.sort(reverse=True)      # sorts in placeprint(scores)

[58, 72, 88, 95]
[72, 95, 58, 88]
[95, 88, 72, 58]

Pass key= to sort by something other than the value itself, such as the length of each word:

words = ["banana", "fig", "apple"]print(sorted(words, key=len))

['fig', 'apple', 'banana']

Copy a list without surprises

Assigning a list to a new name does not copy it. Both names point at the same list, so a change through one name shows up in the other.

a = [1, 2]b = a            # b is the SAME list, not a copyb.append(3)print(a) c = a.copy()     # c is a new listc.append(4)print(a, c)

[1, 2, 3]
[1, 2, 3] [1, 2, 3, 4]

Check your understanding

x = [1, 2, 3]y = x * 2print(y, len(y))

[1, 2, 3, 1, 2, 3] 6

["a@x.com", "b@x.com", "a@x.com", "c@x.com", "b@x.com"]

['a@x.com', 'b@x.com', 'c@x.com']

emails = ["a@x.com", "b@x.com", "a@x.com", "c@x.com", "b@x.com"] unique = list(dict.fromkeys(emails))print(unique)

['a@x.com', 'b@x.com', 'c@x.com']

Program Practice: Python list programs

Practise what you just learned. Start with the easy programs and work up.

Python lists FAQ