Make a good use of the EmptySetError in EnumeratedSets and Parent

[sagetrac-nborie]

As Florent Hivert gave extremely useful features about empty things, we should use it. The Following should returns an EmptySetError

sage: S = Set([])
sage: S.an_element()
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)

...


StopIteration: 
sage: 

Also, make the EmptySetError being documented already in the structure of Parent. Improve the documentation of an_element and _an_element_ in Parent.

CC: @sagetrac-sage-combinat

Component: categories

Keywords: days28, empty set, EmptySetError

Author: Nicolas Borie, Nicolas M. Thiéry

Reviewer: Paul Zimmermann, Robert Bradshaw, Nicolas Borie, Nicolas M. Thiéry

Merged: sage-4.7.alpha3

Issue created by migration from https://trac.sagemath.org/ticket/10651

[slel]

Changed keywords from empty set, EmptySetError to days28, empty set, EmptySetError

[robertwb]

comment:3

Technically, some_elements should return an iterable, not an iterator, so you can iterate over it more than once. The except clause should be changed to return [], not return iter([]), though with the empty case the behavior is the same.

[sagetrac-nborie]

comment:4

Good point. I will ask today Nicolas what does he think about that... I don't really know the specifications for the Test Suite and how deal exactly with such corner case.
Florent did this with FiniteEnumeratedSet (The user know a priori that the set is finite...)

sage: FiniteEnumeratedSet([])
{}
sage: FiniteEnumeratedSet([]).some_elements()
<generator object _some_elements_from_iterator at 0xb433464>

I will give an answer shortly for that.

[sagetrac-nborie]

comment:5

As this ticket fix a corner case and try to increase of the coherence for the code about sets, it have to verify all the specifications. Thanks you very much Robert for pointing this. It now return []. Even the behavior is the same, I don't want to fix a mathematical corner case by introducing a Spec corner case...

It should be ready for review now.

[zimmermann6]

comment:6

I confirm the problem is fixed, and all doctests still pass.
Thus I give a positive review.

Paul

[zimmermann6]

Reviewer: Paul Zimmermann

[zimmermann6]

Author: Nicolas Borie