As you probably recall, dictionaries differ from lists in that you can access items in a dictionary by a key rather than a position.
The thing that is most important to notice right now is that the
get item and
set item operations on a dictionary are O(1). Another important dictionary operation is the
contains operation. Checking to see whether a key is in the dictionary or not is also O(1).
One important side note on dictionary performance is that the efficiencies we provide in the table are for average performance. In some rare cases the
get item, and
set item operations can degenerate into O(N) performance.