sets anomaly

Wed Dec 7 10:33:59 EST 2016

On Wednesday, December 7, 2016 at 10:18:32 AM UTC-5, Rustom Mody wrote:
> Trying to write some code using sets (well frozen sets)
> And was hit by this anomaly
>> This is the behavior of lists I analogously expect in sets:
>> >>> []
> []
> >>> [[]]
> [[]]
> >>> 
>> ie the empty list and the list of the empty list are different things
>> However
> (with
> f= frozenset
> )
>> >>> f()
> frozenset()
> >>> f([])
> frozenset()
> >>> f(f([]))
> frozenset()
> >>> 

The difference is more about the difference between the behavior of a
callable constructor, and a list literal. Lists will behave the same
as frozenset if you use list():
 >>> list()
 []
 >>> list(list())
 []
 >>> list(list(list()))
 []
This is because the constructors can take a sequence, and use those
elements as the new contents. List literals don't work that way.
> Spent a good ½ hour finding out this strangeness
> And then some figuring out how to get an empty set into a set
> This is the best I get:
> >>> f([f([])])
> frozenset({frozenset()})

That is the way I would have done it also. Or:
 >>> s = set()
 >>> s.add(frozenset())
 >>> frozenset(s)
 frozenset([frozenset([])])
Notice the repr output of the result shows how to make it! :)
--Ned.