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Re: Guido rethinking removal of cmp from sort method

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  Re: Guido rethinking removal of cmp from sort method MRAB <python@mrabarnett.plus.com> - 2011-03-30 00:06 +0100

#2198 — Re: Guido rethinking removal of cmp from sort method

On 29/03/2011 21:32, Dan Stromberg wrote:
>
> On Tue, Mar 29, 2011 at 12:48 PM, Ian Kelly <ian.g.kelly@gmail.com
> <mailto:ian.g.kelly@gmail.com>> wrote:
>
>     On Tue, Mar 29, 2011 at 1:08 PM, Chris Angelico <rosuav@gmail.com
>     <mailto:rosuav@gmail.com>> wrote:
>      > On Wed, Mar 30, 2011 at 5:57 AM, MRAB <python@mrabarnett.plus.com
>     <mailto:python@mrabarnett.plus.com>> wrote:
>      >> You would have to do more than that.
>      >>
>      >> For example, "" < "A", but if you "negate" both strings you get "" <
>      >> "\xBE", not "" > "\xBE".
>      >
>      > Strings effectively have an implicit character at the end that's less
>      > than any other character. Easy fix: Append a character that's greater
>      > than any other. So "" < "A" becomes "\xFF" > "\xBE\xFF".
>      >
>      > Still not going to be particularly efficient.
>
>     Not to mention that it still has bugs:
>
>     "" < "\0"
>     "\xff" < "\xff\xff"
>     --
>     http://mail.python.org/mailman/listinfo/python-list
>
>
> It probably could be a list of tuples:
>
> 'abc' -> [ (1, chr(255 - ord('a')), (1, chr(255 - ord('b')), (1, chr(255
> - ord('c')), (0, '') ]
>
> ...where the (0, '') is an "end of string" marker, but it's getting
> still slower, and quite a lot bigger, and getting so reductionistic
> isn't a good thing when one class wraps another wraps another - when
> their comparison methods are interrelated.
>
I think I've found a solution:

     class NegStr:
         def __init__(self, value):
             self._value = value
         def __lt__(self, other):
             return self._value > other._value

so when sorting a list of tuples:

     def make_key(t):
         return NegStr(t[0]), t[1]

     items = [("one", 1), ("two", 2), ("three", 3), ("four", 4), 
("five", 5)]
     items.sort(key=make_key)

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