Modern Python Standard Library Cookbook

How it works...

The ChainMap class receives multiple dictionaries as arguments; whenever a key is requested to ChainMap, it's actually going through the provided dictionaries one by one to check whether the key is available in any of them. Once the key is found, it is returned, as if it was a key owned by ChainMap itself.

The default value for options that are not provided is implemented by having defaultdict as the last dictionary provided to ChainMap. Whenever a key is not found in any of the previous dictionaries, it gets looked up in defaultdict, which uses the provided factory function to return a default value for all keys.