Using ChainMap for defaults and overrides

We'll often have a configuration file hierarchy. Previously, we listed several locations where configuration files can be installed. The configparser module, for example, is designed to read a number of files in an order and integrate the settings by having later files override values from earlier files.

We can implement an elegant default-value processing using the collections.ChainMap class. See Chapter 6, Creating Containers and Collections, for some background on this class. We'll need to keep the configuration parameters as dict instances, which is something that works out well using exec() to evaluate Python-language initialization files.

Using this will require us to design our configuration parameters as a flat dictionary of values. This may be a bit of a burden for applications with a large number of complex configuration values that are integrated from several sources. We'll show you a sensible way to flatten names.

First, we'll build a list...