Bioinformatics Programming Using Python

Chapter 3. Collections

Instances of primitive Python types represent individual values, as we saw in the first chapter. Python also has built-in types that are compound, meaning that they group together multiple objects. These are called collections or containers. The objects in a collection are generally referred to as items or elements. Collections can hold any number of items. Some collection types can even contain items with a mixture of types, including other collections.

There are three categories of built-in collection types in Python: sets, sequences, and mappings. Streams are another kind of collection whose elements form a series in time, rather than the computer’s memory space. Though they are not formally recognized in Python, many of Python’s functions and methods—especially those that manipulate files—implement stream-like behavior. Modules such as array and collections provide additional collection types.

The primary distinguishing characteristic of the four collection categories is how individual elements are accessed: sets don’t allow individual access; sequences use numerical indexes; mappings use keys; and streams just provide one address—“next.” Strings straddle the border between primitives and collections, having characteristics of both; when treated as collections they are interpreted as sequences of one-character substrings.

Some types are immutable, meaning that once they are created they cannot be changed: elements may not be added, removed, or reordered. ...