Indexable weak ordered set in Python

Question

I was wondering if there is an easy way to build an indexable weak ordered set in Python. I tried to build one myself. Here\'s what I came up with:

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Answer 1

Raymond has a great and succinct answer, as usual, but I actually came here a while back interested in the indexable part, more than the weakref part. I eventually built my own answer, which became the IndexedSet type in the boltons utility library. Basically, it's all the best parts of the list and set APIs, combined.

>>> x = IndexedSet(list(range(4)) + list(range(8)))
>>> x
IndexedSet([0, 1, 2, 3, 4, 5, 6, 7])
>>> x - set(range(2))
IndexedSet([2, 3, 4, 5, 6, 7])
>>> x[-1]
7
>>> fcr = IndexedSet('freecreditreport.com')
>>> ''.join(fcr[:fcr.index('.')])
'frecditpo'

If the weakref part is critical you can likely add it via inheritance or direct modification of a copy of the code (the module is standalone, pure-Python, and 2/3 compatible).