These represent finite ordered sets indexed by non-negative
numbers. The built-in function len returns the number of items of a
sequence. When the length of a sequence is n, the index set contains
the numbers 0, 1, …, n-1. Item i of sequence a is selected by
Sequences also support slicing: “a[i:j]” selects all items with index k such that i <= k < j. When used as an expression, a slice is a sequence of the same type. This implies that the index set is renumbered so that it starts at 0.
Some sequences also support
extended slicing with a third step
a[i:j:k] selects all items of a with index x where
x = i + n*k, n >= 0 and i <= x < j.
Sequences are distinguished according to their mutability.
An object of an immutable sequence type cannot change once it is created. (If the object contains references to other objects, these other objects may be mutable and may be changed; however, the collection of objects directly referenced by an immutable object cannot change.)