List Syntax#

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. Try it live and convert a string of your own:

>>> chars = list("abc")
>>> print(chars)
['a', 'b', 'c']

Indexing and Length#

Elements are accessed by integer index starting at 0. Try it live and index into the list yourself:

>>> nums = [4, 7, -2, 5]
>>> print(nums[0])    # first element
4
>>> print(nums[3])    # last element
5
>>> print(len(nums))  # number of elements
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. Try it live and assign to a different index:

>>> nums = [4, 7, -2, 5]
>>> nums[2] = 99
>>> print(nums)
[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. Try it live and edit the slice bounds:

>>> nums = [10, 20, 30, 40, 50]
>>> print(nums[1:4])   # indices 1, 2, 3
[20, 30, 40]
>>> print(nums[:3])    # from the beginning through index 2
[10, 20, 30]
>>> print(nums[2:])    # from index 2 to the end
[30, 40, 50]
>>> print(nums[:])     # a copy of the whole list
[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. Try it live and test for values of your own:

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

Iterating Over a List#

Use a for loop to visit each element. Try it live and edit the words:

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

To iterate with an index, use enumerate(). Try it live:

>>> scores = [88, 73, 95]
>>> for i, score in enumerate(scores):
...     print(f"{i}: {score}")
...
0: 88
1: 73
2: 95

List Syntax

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