Is there a standard binary search with overridable comparisons?
sjmachin at lexicon.net
Wed Jun 18 01:36:32 CEST 2008
On Jun 18, 6:55 am, markscottwright <markscottwri... at gmail.com> wrote:
> I've got an ordered list of MyClasses that I want to be able to do
> binary searches on, but against a tuple. MyClass has valid
> __lt__(self, rhs) and __eq__(self, rhs) member functions that work
> when rhs is a tuple.
> This works:
> l = [MyClass(..), MyClass(..), ...]
> But this doesn't:
> bisect.bisect(l, (a,b))
> I'm assuming
... Don't. It can be dangerous.
> this is because inside bisect, it does 'key < list[x]'
> rather than 'list[x] < key', so it's the tuple's __lt__ that is
> called, rather than MyClass's tuple.
Actually it appears (extremely gory details in Objects/object.c) that
it tries all rich comparison possibilities first:
tuple < myclass: not defined in tuple type
myclass > tuple: not defined in MyClass
before falling through to the default (which is based on addresses).
> Is there a way around this? Can I monkeypatch a new __lt__ into the
> tuple class?
Looks like you need to implement MyClass.__gt__
I suspect someone will say that this section of the manual is trying
to tell us this:
There are no reflected (swapped-argument) versions of these methods
(to be used when the left argument does not support the operation but
the right argument does); rather, __lt__() and __gt__() are each
other's reflection, __le__() and __ge__() are each other's reflection,
and __eq__() and __ne__() are their own reflection.
... "trying" being the operative word :-)
Alternatively, do you really need rich comparison? Consider defining
__cmp__ instead of 2 to 6 of the __lt__ etc brigade.
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