DisjointSet ignores errors

[videlec]

Because DisjointSet is a cython class, some care has to be taken with respect to functions that contain python code that could possibly raise an error. For example, the function union does contain python code that could possibly raise an error, but the error is not raised

sage: d = DisjointSet('abcde')
sage: d.union('a', 'f')
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
KeyError: 'f'
Exception ignored in: 'sage.sets.disjoint_set.DisjointSet_of_hashables.union'
Traceback (most recent call last):
  File "<ipython-input-8-206901714f0d>", line 1, in <module>
KeyError: 'f'

[DaveWitteMorris]

Please clarify why this is a bug. I think raising an error is correct here (since 'f' is not one of the sets).

[videlec]

The bug is that the error is not raised. Isn't it explicit enough?

[DaveWitteMorris]

OK, I was confused. (My bad, but github issues usually include both the expected behavior and the actual behavior, and I was confused about both in this case.) If I understand correctly, currently the code prints a message but does not raise an error, but you (like me) think it should raise an error.

[videlec]

Indeed. The code prints that an error has occurred and been ignored. And I think it should raise an error.

[dcoudert]

method union is currently defined with a noexcept. We can change that to except *. I will push a possible fix. Let's hope it will not break too many tests.