Dictionary Views, Sorting and Merging

Dynamic Dictionary Views

Dictionary view is a live window onto a dictionary's keys, values or items. It changes when the dictionary changes and copies nothing.

In Python 3, .keys(), .values(), and .items() return dynamic view objects rather than static lists.

Views consume O(1) memory because they reference the underlying dictionary directly.

Set Operations on Dictionary Keys

Key view is the object returned by d.keys(). It behaves like a set, so you can use &, | and - on it.

Dictionary .keys() views behave like set objects, supporting set union, intersection, and difference directly!

This eliminates the need to explicitly convert keys to set(d.keys()).

Sorting Dictionaries by Value

Key function is a function passed as key= to sorted(). It returns the value that sorted() should compare for each item.

Use sorted() on .items(), passing a lambda function extracting index 1 (the value).

dict(sorted_items) keeps that order, because dictionaries remember insertion order. New keys added later go at the end. Sorting costs O(N log N).

Dictionary Merge Operators (| and |=)

Union operator is the | symbol on two dictionaries. It builds a new dictionary, and the right side wins when keys repeat.

Introduced in PEP 584 (Python 3.9), the | operator cleanly merges two dictionaries.

Use d1 |= d2 for in-place updating.

Comparing two snapshots

Key views make a change report short: added, removed and changed keys come from set operations. View creation is O(1); each operation here is O(N).

Checking key in d.keys() is O(1), but value in d.values() scans every value in O(N), so keep lookups on the keys.