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python: how to sort a complex list on two different keys

Answer 1

A:

It doesn't need to be a lambda function you pass into the sort method, you can actually provide a real function since they are first-class objects in python.

L.sort(my_comparison_function)

Should work just fine

Daniel DiPaolo 2010-10-20 16:04:18

-1 Comparison function goes away in Python 3, whereas the other answer is future proof.

Steven Rumbalski 2010-10-20 16:12:24

I thought about this, but how can I write a comparison function that works on two different keys? basically I need a f((x[0],x[1]), (y[0],y[1]))

Giovanni Di Milia 2010-10-20 16:14:05

@Steven: what are you talking about? this answer might be useless, but not for the reasons you state. [Read the docs](http://docs.python.org/py3k/library/stdtypes.html#mutable-sequence-types)

SilentGhost 2010-10-20 16:14:54

@SilentGhost Maybe I'm misreading, but when I look at the linked docs i see that sort accepts a /key/ function in python 3, but not a /comparison/ function.

Steven Rumbalski 2010-10-20 17:24:48

@Steven: and what is key function if not a comparison function?

SilentGhost 2010-10-20 17:26:37

@SilentGhost A comparison function is called on pairs of items and "should return a negative integer if self < other, zero if self == other, a positive integer if self > other." Whereas a key function is called once for each item and returns the value(s) you want an item sorted on. See http://wiki.python.org/moin/HowTo/Sorting/ paying attention specifically to sections titled _Key_Functions_ and _The_Old_Way_Using_the_cmp_Parameter_. The key parameter was added in Python 2.4. The cmp parameter was removed in Python 3.

Steven Rumbalski 2010-10-20 18:05:51

Answer 2

+4 A:

This should do it:

l=[[name_d, 5], [name_e, 10], [name_a, 5]]
sorted(l,key=lambda x: (x[1]*-1,x[0]))

Edit: Sort order

Till Backhaus 2010-10-20 16:04:40

he needs descending sorting for the number

SilentGhost 2010-10-20 16:07:00

this doesn't work in the way I want, because it sorts also the name in desc (or asc) mode

Giovanni Di Milia 2010-10-20 16:09:20

@Giovanni: it's not a black box. Do you see how it is solved? and you cannot modify it to suit your problem?

SilentGhost 2010-10-20 16:10:26

yeah, `-x[1]` would do

SilentGhost 2010-10-20 16:10:55

Corrected the sort order. Like SilentGhost said already - it should have been obvious how to change the order.

Till Backhaus 2010-10-20 16:15:04

btw: if you don't want to copy the listL `l.sort(key=lambda...)`

Till Backhaus 2010-10-20 16:15:40

@SilentGhost, Sorry, I didn't know that I could invert the sorting with the "-"

Giovanni Di Milia 2010-10-20 16:18:31

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