Python offers several stateful collections; the various mappings include the dict class and a number of related mappings defined in the collections module. We need to emphasize the stateful nature of these mappings and use them carefully.
For our purposes, learning functional programming techniques in Python, there are two use cases for mapping: a stateful dictionary that accumulates a mapping and a frozen dictionary. In the first example of this chapter, we showed a frozen dictionary that was used by the ElementTree.findall() method. Python doesn't provide an easy-to-use definition of an immutable mapping. The collections.Mapping abstract class is immutable, but it's not something we can use trivially. We'll dive ...
