Lists in Python

Real programs work with many values at once: the items in a cart, the lines of a file, the scores of a class. The basic tool is the list, which the docs call the most versatile of Python’s compound data types: values in square brackets, in a fixed order, that you can change.

Every example here was run with Python 3.13. The behavior was checked against the lists section of the tutorial, more on lists and the list type reference.

Creating a list

Write the values between square brackets, separated by commas:

fruits = ["apple", "banana", "cherry"]
scores = [90, 72, 85]
empty = []
mixed = ["Dana", 30, True]
print(fruits, len(fruits))
print(empty, len(empty))
print(type(scores))

['apple', 'banana', 'cherry'] 3
[] 0
<class 'list'>

A list can mix types, like mixed, but usually all items share one type. len() gives the number of items, as for a string.

list() builds a list from anything you can go through item by item, like a string (type conversion post) or a range (loops post).

Indexing and slicing

Lists use the same indexes as strings: the first item is 0, and negative indexes count from the end. A slice [start:stop] returns a new list:

fruits = ["apple", "banana", "cherry", "date"]
print(fruits[0], fruits[-1])
print(fruits[1:3])
print(fruits[::-1])

apple date
['banana', 'cherry']
['date', 'cherry', 'banana', 'apple']

An index past the end is an error, but a slice past the end returns what exists:

print(fruits[1:100])
print(fruits[10])

['banana', 'cherry', 'date']
IndexError: list index out of range

Changing a list

Here lists differ from strings: a string can’t be changed, but a list can (the tutorial: “lists are a mutable type”). You can replace an item in place:

scores = [90, 72, 85]
scores[1] = 75
print(scores)

[90, 75, 85]

To add items, use methods. append() adds one item to the end, insert() adds one at a position, and extend() adds every item of another list:

cart = ["milk"]
cart.append("bread")
cart.insert(0, "eggs")
cart.extend(["tea", "rice"])
print(cart)

['eggs', 'milk', 'bread', 'tea', 'rice']

To remove items, remove() deletes the first item with a given value, pop() deletes by position and returns the item, and del deletes by index or slice:

cart.remove("milk")
last = cart.pop()
first = cart.pop(0)
print(cart, last, first)

del cart[0]
print(cart)

['bread', 'tea'] rice eggs
['tea']

pop() with no argument takes from the end. That makes a list a natural stack: append() to push, pop() to take the last item back.

Searching and counting

in checks if a value is in the list and returns True or False. index() gives the position of the first match, and count() counts matches:

colors = ["red", "green", "blue", "green"]
print("green" in colors, "pink" in colors)
print(colors.index("green"), colors.count("green"))

True False
1 2

index() raises an error when the value isn’t there, so check with in first (see conditions):

print(colors.index("pink"))

ValueError: 'pink' is not in list

For lists of numbers, sum(), min() and max() do what their names say:

scores = [90, 72, 85]
print(sum(scores), min(scores), max(scores))
print(sum(scores) / len(scores))

247 72 90
82.33333333333333

Sorting

There are two ways to sort. The sort() method sorts the list itself and returns None. The sorted() function leaves the list alone and returns a new, sorted list:

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

numbers.sort()
print(numbers)

numbers.sort(reverse=True)
print(numbers)

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

The key argument sorts by something computed from each item. key=len sorts words by length, and key=str.lower ignores case:

words = ["banana", "Apple", "fig"]
print(sorted(words))
print(sorted(words, key=str.lower))
print(sorted(words, key=len))

['Apple', 'banana', 'fig']
['Apple', 'banana', 'fig']
['fig', 'Apple', 'banana']

Plain sorted(words) puts "Apple" first because uppercase letters come before lowercase ones, which is rarely what you want for names.

Two names, one list

Assigning a list to another variable doesn’t copy it. Both names point to the same list, so a change through one name shows up through the other:

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

[1, 2, 3, 4]
True

To get an independent list, copy it with copy(), list() or a full slice [:]:

a = [1, 2, 3]
c = a.copy()
c.append(4)
print(a, c)

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

These are shallow copies: a list inside the list is still shared. For numbers and strings, that never matters.

Tuples: a list that can’t change

A tuple is written with parentheses instead of brackets. It’s indexed like a list, but it can’t be changed after it’s created:

point = (3, 4)
print(point[0], len(point))
x, y = point
print(x, y)
point[0] = 10

3 2
3 4
TypeError: 'tuple' object does not support item assignment

x, y = point is unpacking: one name per item, as in x, y = 1, 2 from the variables post. Use a tuple for a fixed group that belongs together, like coordinates, and a list for a collection that grows and shrinks.

Common mistakes

• Using the result of sort(). numbers = numbers.sort() sets numbers to None. Use numbers.sort() alone, or numbers = sorted(numbers).
• append() with a list. cart.append(["tea", "rice"]) adds one item, a list inside the list. To add each item, use extend().
• Off-by-one indexes. The last item is items[len(items) - 1], or simply items[-1]. items[len(items)] is an IndexError.
• Calling a variable list. list = [1, 2] hides the built-in list() (shadowing).

What’s next

Next: conditions with if, elif and else, which decide what to run based on values like "green" in colors.