List Syntax

Note

Source: Adapted from the C# edition (arrays/onedim.rst and lists/listsyntax.rst). Python lists replace both C# fixed-size arrays and List<T>. Python lists are mutable, dynamically sized, and can hold elements of mixed types.

A list is an ordered, mutable sequence of values. It is the most commonly used collection in Python.

Creating Lists

Use square brackets to write a list literal:

nums = [4, 7, -2, 5]
words = ["up", "down", "over"]
empty = []
mixed = [1, "hello", 3.14]   # mixed types are allowed

list() converts any iterable to a list:

chars = list("abc")
print(chars)

Output:

['a', 'b', 'c']

Indexing and Length

Elements are accessed by integer index starting at 0:

nums = [4, 7, -2, 5]
print(nums[0])    # first element
print(nums[3])    # last element
print(len(nums))  # number of elements

Output:

4
5
4

Negative indices count from the end: nums[-1] is the last element, nums[-2] is the second-to-last, and so on.

Mutation

Unlike strings, lists can be changed after creation:

nums = [4, 7, -2, 5]
nums[2] = 99
print(nums)

Output:

[4, 7, 99, 5]

Slicing

A slice extracts a sub-list. The syntax is lst[start:stop], which returns elements from index start up to (but not including) stop:

nums = [10, 20, 30, 40, 50]
print(nums[1:4])   # indices 1, 2, 3
print(nums[:3])    # from the beginning through index 2
print(nums[2:])    # from index 2 to the end
print(nums[:])     # a copy of the whole list

Output:

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

A slice always produces a new list; the original is unchanged.

Membership Test

The in operator tests whether a value is anywhere in the list:

words = ["up", "down", "over"]
print("down" in words)
print("around" in words)

Output:

True
False

Iterating Over a List

Use a for loop to visit each element:

for word in ["cat", "dog", "bird"]:
    print(word.upper())

Output:

CAT
DOG
BIRD

To iterate with an index, use enumerate():

scores = [88, 73, 95]
for i, score in enumerate(scores):
    print(f"{i}: {score}")

Output:

0: 88
1: 73
2: 95