An OrderedSet is a custom MutableSet that remembers its order, so that every entry has an index that can be looked up.

Based on a recipe originally posted to ActiveState Recipes by Raymond Hettiger, and released under the MIT license:

http://code.activestate.com/recipes/576694-orderedset/

Rob Speer's changes are as follows:

• changed the content from a doubly-linked list to a regular Python list. Seriously, who wants O(1) deletes but O(N) lookups by index?
• add() returns the index of the added item
• index() just returns the index of an item
• added a __getstate__ and __setstate__ so it can be pickled
• added __getitem__
• __getitem__ and index() can be passed lists or arrays, looking up all the elements in them to perform NumPy-like "fancy indexing"

minghu6's changes are as follow:

• restrict the OrderedSet operation object: only themselves. Because OrderededSet's element consists of its index and value, Python set's element only consists of its value, however. I have written a new class OrderedSetAdapter to adapt Python set.
• writtern a new class OrderedSetAdapter
• rewrittern some contradictory method from collections.MutableSet

Tested on Python 2.7, 3.3, 3.4, 3.5, 3.6, PyPy, and PyPy3.