Dictionaries and sets

Stop scanning lists

Checking membership in a list walks it from the start; checking membership in a set or a dict is effectively constant time. Converting a lookup list to a set is often the entire fix for a script that got slow as its input grew.

Dictionaries preserve insertion order and are the natural shape for records keyed by id. Grouping is the most common operation you will write, and a default dictionary removes the does-this-key-exist boilerplate.

Set algebra — union, intersection, difference — answers which-items-are-in-both questions directly, without any loop at all.