A list value is a sequence that can have arbitrary length. Lists have a large set of built-in behaviors, along with specific syntax to express those behaviors. We have already seen the list literal, which evaluates to a list instance, as well as an element selection expression that evaluates to a value in the list. The built-in len function returns the length of a sequence. Below, digits is a list with four elements. The element at index 3 is 8.

>>> digits = [1, 8, 2, 8]
>>> len(digits)
4
>>> digits[3]
8

Additionally, lists can be added together and multiplied by integers. For sequences, addition and multiplication do not add or multiply elements, but instead combine and replicate the sequences themselves. That is, the add function in the operator module (and the + operator) yields a list that is the concatenation of the added arguments. The mul function in operator (and the * operator) can take a list and an integer k to return the list that consists of k repetitions of the original list.

>>> [2, 7] + digits * 2
[2, 7, 1, 8, 2, 8, 1, 8, 2, 8]

Any values can be included in a list, including another list. Element selection can be applied multiple times in order to select a deeply nested element in a list containing lists.

>>> pairs = [[10, 20], [30, 40]]
>>> pairs[1]
[30, 40]
>>> pairs[1][0]
30