Does Python have an ordered set?

Question

Python has an ordered dictionary. What about an ordered set?

Answer 1

Implementations on PyPI

While others have pointed out that there is no built-in implementation of an insertion-order preserving set in Python (yet), I am feeling that this question is missing an answer which states what there is to be found on PyPI.

There are the packages:

• ordered-set (Python based)
• orderedset (Cython based)
• collections-extended
• boltons (under iterutils.IndexedSet, Python-based)
• oset (last updated in 2012)

Some of these implementations are based on the recipe posted by Raymond Hettinger to ActiveState which is also mentioned in other answers here.

Some differences

• ordered-set (version 1.1)
• advantage: O(1) for lookups by index (e.g. my_set[5])
• oset (version 0.1.3)
• advantage: O(1) for remove(item)
• disadvantage: apparently O(n) for lookups by index

Both implementations have O(1) for add(item) and __contains__(item) (item in my_set).