ChainMap

A ChainMap groups multiple dicts (or other mappings) together
to create a single, updateable view.

The underlying mappings are stored in a list.  That list is public and can
be accessed or updated using the *maps* attribute.  There is no other
state.

Lookups search the underlying mappings successively until a key is found.
In contrast, writes, updates, and deletions only operate on the first
mapping.