On Jun 22, 2009, at 2:25 AM, Franco Saliola wrote:
>
> Hello,
>
> Here is a quick description of what is below: Subclasses of Element
> complain that no sorting algorithm is defined even when all the rich
> comparison methods have been implemented. Bug?
>
> In the code sample below, C is a class that inherits from Element and
> implements all the rich comparison methods.
>
> {{{
> sage: class C(sage.structure.element.Element):
> ... def __init__(self, a):
> ... self.a = a
> ... def __repr__(self):
> ... return str(self.a)
> ... def __eq__(self, other):
> ... return self.a == other.a
> ... def __ne__(self, other):
> ... return self.a != other.a
> ... def __lt__(self, other):
> ... return self.a < other.a
> ... def __le__(self, other):
> ... return self.a <= other.a
> ... def __gt__(self, other):
> ... return self.a > other.a
> ... def __ge__(self, other):
> ... return self.a >= other.a
> }}}
>
> In theory, the cmp function should use the rich comparison methods for
> comparisions. However, since __cmp__ has also been implemented (it is
> implemented by Element), cmp bypasses all the rich comparison
> methods and
> calls __cmp__ directly. This leads to an error since Element requires
> subclasses to define a sorting algorithm:
>
> {{{
> sage: a = C('a')
> sage: b = C('b')
>
> sage: a < b
> True
>
> sage: cmp(a,b)
> ------------------------------------------------------------
> Traceback (most recent call last):
> File "<ipython console>", line 1, in <module>
> File "element.pyx", line 648, in
> sage.structure.element.Element.__cmp__ (sage/structure/element.c:6064)
> File "element.pyx", line 561, in sage.structure.element.Element._cmp
> (sage/structure/element.c:5135)
> File "element.pyx", line 663, in
> sage.structure.element.Element._cmp_c_impl
> (sage/structure/element.c:6239)
> NotImplementedError: BUG: sort algorithm for elements of 'None' not
> implemented
> }}}
>
> And this leads to problems with sorting via the cmp function:
>
> {{{
> sage: sorted([b,a])
> [a, b]
>
> sage: sorted([b,a], cmp=cmp)
> ------------------------------------------------------------
> Traceback (most recent call last):
> File "<ipython console>", line 1, in <module>
> File "element.pyx", line 648, in
> sage.structure.element.Element.__cmp__ (sage/structure/element.c:6064)
> File "element.pyx", line 561, in sage.structure.element.Element._cmp
> (sage/structure/element.c:5135)
> File "element.pyx", line 663, in
> sage.structure.element.Element._cmp_c_impl
> (sage/structure/element.c:6239)
> NotImplementedError: BUG: sort algorithm for elements of 'None' not
> implemented
> }}}
>
> One can get around this by implementing __cmp__ as well as the rich
> comparison methods, but I'm wondering whether it might be better for
> Element to check for and use the rich comparison methods before
> freaking
> out.
>
> Is there a good reason (speed? efficiency?) for this behaviour?
> Should I
> open a ticket?
I would very much be in favor of simplifying comparisons! There are a
couple reasons the code is the way it is. First, in Cython (well, any
Python Extension class), either __cmp__, __richcmp__ and __hash__ are
all inherited together, or none are. Second, the code was written
before cpdef methods, and had to handle both Python and Cython
classes. Finally, there's just a lot of cruft from all the years it's
gone through, and it's easy to break things mucking around with it so
no one's gone and really cleaned it up. (There was an effort to do so
as part of the late coercion branch, but it was too much on top of
all else that was going on).
I would be in favor of a SEP regarding these issues, as well as
summarizing the long thread we had earlier this year dealing with
incomparable items and whether or not RealField(200)(pi) == RealField
(100)(pi).
- Robert
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