[Tim]
> BTW, what should
>
> {1, 2} | {3, 4, 5, 6, 7}
>
> return as ordered sets? Beats me.;
[Larry]
> The obvious answer is {1, 2, 3, 4, 5, 6, 7}.
Why? An obvious implementation that doesn't ignore performance entirely is:
def union(smaller, larger):
if len(larger) < len(smaller):
smaller, larger = larger, smaller
result = larger.copy()
for x in smaller:
result.add(x)
In the example, that would first copy {3, 4, 5, 6, 7}, and then add 1
and 2 (in that order) to it, giving {3, 4, 5, 6, 7, 1, 2} as the
result.
If it's desired that "insertion order" be consistent across runs,
platforms, and releases, then what "insertion order" _means_ needs to
be rigorously defined & specified for all set operations. This was
comparatively trivial for dicts, because there are, e.g., no
commutative binary operators defined on dicts.
If you want to insist that `a | b` first list all the elements of a,
and then all the elements of b that weren't already in a, regardless
of cost, then you create another kind of unintuitive surprise: in
general the result of "a | b" will display differently than the result
of "b | a" (although the results will compare equal), and despite that
the _user_ didn't "insert" anything.
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