[Python-ideas] cmp and sorting non-symmetric types

[Guido van Rossum]

Are you sure you're solving a real problem?

On Nov 14, 2007 9:49 AM, Adam Olsen <rhamph at gmail.com> wrote:
> (ugh, this was supposed to go to python-ideas, not python-list.  No
> wonder I got no responses to this email!)
>
> (I've had trouble getting response for collaboration on a PEP.
> Perhaps I'm the only interested party?)
>
> Although py3k raises an exception for completely unsortable types, it
> continues to silently do the wrong thing for non-symmetric types that
> overload comparison operator with special meanings.
>
> >>> a = set([1])
> >>> b = set([2, 5])
> >>> c = set([1, 2])
> >>> sorted([a, c, b])
> [{1}, {1, 2}, {2, 5}]
> >>> sorted([a, b, c])
> [{1}, {2, 5}, {1, 2}]
>
> To solve this I propose a revived cmp (as per the previous thread[1]),
> which is the preferred path for orderings.  The rich comparison
> operators will be simple wrappers for cmp() (ensuring an exception is
> raised if they're not merely comparing for equality.)
>
> Thus, set would need 7 methods defined (6 rich comparisons plus
> __cmp__, although it could skip __eq__ and __ne__), whereas nearly all
> other types (int, list, etc) need only __cmp__.
>
> Code which uses <= to compare sets would be assumed to want subset
> operations.  Generic containers should use cmp() exclusively.
>
>
> [1] http://mail.python.org/pipermail/python-3000/2007-October/011072.html
>
> --
> Adam Olsen, aka Rhamphoryncus
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>



-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)