[Tutor] Sorting list of tuples in two passes

[Dayo Adewunmi]

On 08/28/2011 06:23 PM, Hugo Arts wrote:
> On Sun, Aug 28, 2011 at 6:43 PM, Dayo Adewunmi<contactdayo at gmail.com>  wrote:
>> Hi
>>
>> I have a list of tuples that each have four elements:
>>
>> [(firstName,lastName,userName,gidNumber),(.....)....]
>>
>> I'm trying to sort this list in two passes. First by gidNumber and then the
>> subgroups by lastName. So far i've only been able to sort by gidNumber. But
>> I can't seem to wrap my mind around lambda, which is what my browsing around
>> seems to indicate is needed to achieve this?
>>
>> Thanks
>>
>> Dayo
>>
> Python's builtin sort is stable, which means that ordering of items
> with the same key is preserved. This property means that you can do
> multiple pass sorting very easily and efficiently just by sorting
> twice:
>
>>>> # we'll simplify the problem a bit and have tuples with just last name and id.
>>>> l = [('aaa', 1), ('bbb', 1), ('ccc', 1), ('aaa', 2), ('bbb', 2), ('ccc', 2)]
>>>> l.sort(key=itemgetter(0))
>>>> l
> [('aaa', 1), ('aaa', 2), ('bbb', 1), ('bbb', 2), ('ccc', 1), ('ccc', 2)]
>>>> l.sort(key=itemgetter(1))
>>>> l
> [('aaa', 1), ('bbb', 1), ('ccc', 1), ('aaa', 2), ('bbb', 2), ('ccc', 2)]
> We sort by last name first, then sort again by id. As you can see, the
> sorting of groups with the same id is preserved, and our list is now
> in the correct order.
>
> Hugo
>

It works when I use your example, but I don't understand why it won't 
work when I use 4-element tuples instead of 2:

 >>>l = [('wascas','aaa','fdvdfv', 1), ('rtgdsf','bbb','trfg', 1), 
('addwe','ccc','esd', 1), ('xasd','aaa','wascaq', 2), 
('nhy','bbb','asw', 2), ('zzzz','ccc','dgdeg', 2)]
 >>> l
[('wascas', 'aaa', 'fdvdfv', 1), ('rtgdsf', 'bbb', 'trfg', 1), ('addwe', 
'ccc', 'esd', 1), ('xasd', 'aaa', 'wascaq', 2), ('nhy', 'bbb', 'asw', 
2), ('zzzz', 'ccc', 'dgdeg', 2)]
 >>> l.sort(key=itemgetter(3))
 >>> l
[('wascas', 'aaa', 'fdvdfv', 1), ('rtgdsf', 'bbb', 'trfg', 1), ('addwe', 
'ccc', 'esd', 1), ('xasd', 'aaa', 'wascaq', 2), ('nhy', 'bbb', 'asw', 
2), ('zzzz', 'ccc', 'dgdeg', 2)]
 >>> l.sort(key=itemgetter(1))
 >>> l
[('wascas', 'aaa', 'fdvdfv', 1), ('xasd', 'aaa', 'wasca    q', 2), 
('rtgdsf', 'bbb', 'trfg', 1), ('nhy', 'bbb', '
asw', 2), ('addwe', 'ccc', 'esd', 1), ('zzzz', 'ccc', 'dgdeg', 2)]
 >>>


Also I notice your original list and your end result list are in the 
same order.

Thanks

Dayo