OrderedSet.pop() with a non-default index breaks the contract

[rspeer]

>>> from ordered_set import OrderedSet
>>> a = OrderedSet(["a", "b", "c"])
>>> a.index("b")
1
>>> a.pop(0)
'a'
>>> a.index("b")
1
>>> a[1]
'c'

This was revealed by looking into #79. pop() would need to rebuild the mapping in place, like discard() does. Its current behavior, which only works for index=-1, would be an optimization.

[aranega]

Hi @rspeer,

Thanks a lot for your library, it's really really handy! Regarding this point, I implemented some patch in my project to deal with pop/insert and item assignation without slices.

https://github.com/pyecore/pyecore/blob/master/pyecore/ordered_set_patch.py

It's probably not perfect and a little bit naïve implementation, but perhaps it could help.