sort by last then by first

[Padraig at Linux.ie]

Alex Martelli wrote:
> Jørgen Cederberg wrote:
>    ...
> 
>>When sorting a list by several fields, I use this method:
>>
>> >>> names=[['Flintstone', 'Fred'],['Flintstone', 'Wilma'],['Rubble',
>>... 'Barney'],['Rubble', 'Betty'],['Flintstone', 'Pebbles']]
>> >>> snames = [ (x[1], x[0], x) for x in names]
>> >>> snames.sort()
>> >>> sortednames = [x[-1] for x in snames]
>> >>> sortednames
>>[['Rubble', 'Barney'], ['Rubble', 'Betty'], ['Flintstone', 'Fred'],
>>['Flintstone
>>', 'Pebbles'], ['Flintstone', 'Wilma']]
>> >>>
>>
>>I create a list of tuples, where the tuple is ordered in the way I want
>>to sort, i.e. "sort by last then by first". The last element is the
>>original item to be sorted. Sort them, and then extract the original item.
>>
>>Maybe there are faster and easier ways, but I understand this one.
> 
> 
> There is no substantially faster way than this approach (the "Decorate,
> Sort, Undecorate" idiom, aka DSU) for long-enough lists.  There _are_
> several slight variants that can come in handy sometimes, e.g. inserting 
> in each item of sortednames the index into names of the corresponding
> item, in order to make the sort "stable" (items that compare equal on
> all compared fields are then guaranteed to stay in the same order as
> they were originally -- the sort method, per se, is not stable up to
> Python 2.2, and is not guaranteed to remain stable in the future even
> though it is in 2.3).
> 
> I entirely agree that, almost always, DSU is simpler as well as faster
> (when it counts, i.e., for large lists) than the alternatives.  The
> main exceptions come when the sort needs to be ascending on certain
> fields but descending on others -- _then_ DSU gets a bit hairier.

wouldn't it be cool if [].sort() took an optional parameter that
was essentially the --key option in gnu sort. in this e.g:

names.sort("--key=1,1 --key=0,0")

or for more complicated forward and then reverse sorting like you
suggested:

names.sort("--key=1,1r --key=0,0")

Pádraig.