Python: Access members of a set

python, python-3.x, set

Solution

I don't think that `set` supports retrieving an item in O(1) time, but you could use a `dict` instead.

d = {}
d[a] = a
retrieved_a = d[b]

Problem

Say I have a set `myset` of custom objects that may be equal although their references are different (`a == b and a is not b`). Now if I `add(a)` to the set, Python correctly assumes that `a in myset and b in myset` even though there is only `len(myset) == 1` object in the set. That is clear. But is it now possible to extract the value of `a` somehow out from the set, using `b` only? Suppose that the objects are mutable and I want to change them both, having forgotten the direct reference to `a`. Put differently, I am looking for the `myset[b]` operation, which would return exactly the member `a` of the set. It seems to me that the type `set` cannot do this (faster than iterating through all its members). If so, is there at least an effective work-around?

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