[Python-ideas] universal set object for use in set manipulation

[Terry Reedy]

Jan Kaliszewski wrote:
> Mike Meyer 
> <mwm-keyword-python.b4bdba at mired.org>:
> 
>> On the other hand, adding negative sets to the language provides an
>> easy way to express that constant, and certainly have more uses in
>> general. If we had to add one of the two, it'd be these. But I'm not
>> convinced that these have enough uses to justify adding them, either.
> 
> Yeah, I feel, that if a group of people need them, they should have
> a separate type ("negative" or "infitity-minus..." or "universe
> without..."-) sets, in a separate module (not necessarily in std
> library).

It should definitely start on PyPI. Since most of the work of methods 
would be done by the underlying system set (or frozenset) type, a Python 
implementation should be fast enough.
> 
> It would be a type which can interact in some ways with sets (like
> e.g. dict views do...).
> 
> As it was noted -- some sets' features don't make sense here...
> (len, iter).
> 
> But many do:
> 
> a in universe_minus()
> -> True
> 
> a in universe_minus([a, b, c])
> -> False
> 
> set([a, b]) < universe_minus()
> -> True
> 
> set([a, b]) < universe_minus([b, c])
> -> False
> 
> universe_minus().remove(a)
> -> universe_minus([a])
> 
> universe_minus([a, b, c]).remove(a)
> -> KeyError
> 
> universe_minus([a]).add(a)
> -> universe_minus([])
> 
> universe_minus().add(a)
> -> universe_minus([])
> 
> universe_minus() - set([a, b, c])
> -> universe_minus([a, b, c])
> 
> universe_minus() & set([a, b, c])
> -> set([a, b, c])
> 
> universe_minus() | set([a, b, c])
> -> universe_minus()
> 
> universe_minus() - universe_minus([a, b])
> -> set([a, b])
> 
> universe_minus() & universe_minus([a, b])
> -> universe_minus([a, b])
> 
> universe_minus() | universe_minus([a, b])
> -> universe_minus()
> 
> universe_minus([a, b, c]) - set([a, b])
> -> universe_minus([a, b, c])
> 
> universe_minus([a, b, c]) | set([a, b])
> -> universe_minus([c])
> 
> universe_minus([a, b]) ^ set([a, b, c])
> -> universe_minus([c])
> 
> etc.