#5280: [with patch, positive review] problem with a subposet coming from an
order_filter
---------------------------+------------------------------------------------
Reporter: jhpalmieri | Owner: somebody
Type: defect | Status: new
Priority: minor | Milestone: sage-combinat
Component: combinatorics | Keywords:
Reviewer: | Author: Franco Saliola
Merged: |
---------------------------+------------------------------------------------
Comment(by saliola):
Replying to rlm:
> Why is {{{__cmp__}}} returning 1 when elements are incomparable?
Shouldn't it be raising an error instead?
Here are a couple of reasons why it shouldn't.
(1) {{{__cmp__}}} should never raise an error, otherwise you won't be able
to
sort a list of elements:
{{{
sage: class C(object):
... def __cmp__(self, other):
... raise ValueError, 'elements are incomparable'
sage: sorted([C(), C()])
------------------------------------------------------------
Traceback (most recent call last):
File "<ipython console>", line 1, in <module>
File "<ipython console>", line 3, in __cmp__
ValueError: elements are incomparable
}}}
(2) All the rich comparisons have been implemented for {{{PosetElement}}},
so
{{{x<y}}} is handled by {{{x.__lt__(y)}}}. That is, the answer will be
correct.
So you might wonder why {{{__cmp__}}} even needs to be implemented.
Shouldn't
{{{cmp}}} just use the rich comparison methods to determine its value?
In theory, yes. But {{{PosetElement}}} inherits from {{{Element}}}, which
defines {{{__cmp__}}}. It seems to be that since {{{__cmp__}}} is not the
default implementation ({{{object.__cmp__}}}), the {{{cmp}}} function
ignores
all the rich comparison operations and calls {{{__cmp__}}} directly. See
the
following example, which was adapted from
[http://docs.sympy.org/_sources/python-comparisons.txt].
{{{
sage: class C_without_cmp(SageObject):
... def __init__(self, a):
... self.a = a
... def __repr__(self):
... return str(self.a)
... def __eq__(self, o):
... print "%s.__eq__(%s)" % (self.a, o.a)
... return NotImplemented
... def __ne__(self, o):
... print "%s.__ne__(%s)" % (self.a, o.a)
... return NotImplemented
... def __lt__(self, o):
... print "%s.__lt__(%s)" % (self.a, o.a)
... return NotImplemented
... def __le__(self, o):
... print "%s.__le__(%s)" % (self.a, o.a)
... return NotImplemented
... def __gt__(self, o):
... print "%s.__gt__(%s)" % (self.a, o.a)
... return NotImplemented
... def __ge__(self, o):
... print "%s.__ge__(%s)" % (self.a, o.a)
... return NotImplemented
# cmp uses the rich comparison methods if no __cmp__ is found
sage: a = C_without_cmp("a"); b = C_without_cmp("b")
sage: cmp(a,b)
a.__eq__(b)
b.__eq__(a)
b.__eq__(a)
a.__eq__(b)
a.__lt__(b)
b.__gt__(a)
b.__gt__(a)
a.__lt__(b)
a.__gt__(b)
b.__lt__(a)
b.__lt__(a)
a.__gt__(b)
1
sage: class C_with_cmp(C_without_cmp):
... def __cmp__(self, o):
... print "%s.__cmp__(%s)" % (self.a, o.a)
... return NotImplemented
# cmp uses __cmp__, ignoring the rich comparison methods if it is defined
sage: a = C_with_cmp("a"); b = C_with_cmp("b")
sage: cmp(a,b)
a.__cmp__(b)
b.__cmp__(a)
-1
}}}
This leads to the following error for posets, which is what this ticket is
about.
{{{
sage: P = Poset([[1,2],[3],[3],[]])
sage: sorted(P)
[0, 1, 2, 3]
sage: sorted(P, 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:6062)
File "element.pyx", line 561, in sage.structure.element.Element._cmp
(sage/structure/element.c:5133)
File "element.pyx", line 663, in
sage.structure.element.Element._cmp_c_impl (sage/structure/element.c:6237)
NotImplementedError: BUG: sort algorithm for elements of 'Finite poset
containing 4 elements' not implemented
>
/home/saliola/Applications/sage-4.0.2-busted/local/bin/element.pyx(663)sage.structure.element.Element._cmp_c_impl
(sage/structure/element.c:6237)()
}}}
So I implemented {{{__cmp__}}} for {{{PosetElement}}}.
Are these satisfactory reasons?
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/5280#comment:9>
Sage <http://sagemath.org/>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
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