Removing from dictionaries – the pop() method and the del statement

A common use case for a dictionary is as an associative store: we can keep an association between key and value objects. This means that we may be doing any of the CRUD operations on an item in the dictionary.

• Create a new key and value pair
• Retrieve the value associated with a key
• Update the value associated with a key
• Delete the key (and value) from the dictionary

We have two common variations on this theme:

• We have the in-memory dictionary, dict, and the variations on this theme in the collections module. The collection only exists while our program is running.
• We also have persistent storage in the shelve and dbm modules. The data collection is a persistent file in the file system.

These are very similar, the distinctions between a shelf.Shelf and dict object are minor. This allows us to experiment with a dict and switch to a Shelf without making dramatic changes to a program.

A server process will often have multiple, concurrent...