Learn Web Development with Python

ChainMap

ChainMap is an extremely nice data type which was introduced in Python 3.3. It behaves like a normal dictionary but according to the Python documentation: "is provided for quickly linking a number of mappings so they can be treated as a single unit". This is usually much faster than creating one dictionary and running multiple update calls on it. ChainMap can be used to simulate nested scopes and is useful in templating. The underlying mappings are stored in a list. That list is public and can be accessed or updated using the maps attribute. Lookups search the underlying mappings successively until a key is found. By contrast, writes, updates, and deletions only operate on the first mapping.

A very common use case is providing defaults, ...