Bar Harel writes:
> The original example of "if dict.setdefault()" is problematic as well for
> the exact same reason. You can't tell if the default value was already
> there in the first place.
That's not a problem. In both cases if you need an object that can't
possibly be there, you have
# 'default' by analogy to dict.setdefault. It's a bad name for
# this argument to add_unique, and does not have the semantics of
# inserting default in the set.
olditem = someset.add_unique(thing, default=(sentinel:=object()))
if olditem is not sentinel:
# handle dupe
More likely you trust yourself not to insert it, and use a sentinel
defined previously.
Of course, we may prefer a boolean return, despite the general rule
about returning elements. I'm single-threaded, and so agnostic on
that. :-) But if it turns out that somebody *wants* to check "2 is 2.0",
this .add_unique can serve both purposes.
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