For anyone wanting some more explanation, ChainMap can be used to build nested namespaces from a series of dicts without having to explicitly merge the names in each level. Updates to the whole ChainMap go into the top-level dict.

The docs are here [0].

Some simple motivating applications:

- Look up names in Python locals before globals before built-in functions: `pylookup = ChainMap(locals(), globals(), vars(builtins))`

- Get config variables from various sources in priority order: `var_map = ChainMap(command_line_args, os.environ, defaults)`

- Simulate layered filesystems

- etc

[0] https://docs.python.org/3/library/collections.html#collectio...

I found it perfect for structured logging, where you might want to modify some details of the logged structures (e.g. a password) without changing the underlying data.