Is there an easy way to sort a list by two criteria?

[Steven D'Aprano]

On Sat, 09 Feb 2008 18:05:14 -0800, neocortex wrote:

> Hello!
> I am a newbie in Python. Recently, I get stuck with the problem of
> sorting by two criteria. In brief, I have a two-dimensional list (for a
> table or a matrix). Now, I need to sort by two columns, but I cannot
> figure out how to do that.


Can you give a (small, simple) example of your data, and what you expect 
if you sort it successfully?


> I read somewhere that it is possible to do:
> >>> table.sort().sort()
> but it does not work.

No it doesn't, because the sort() method returns None, and None doesn't 
have a sort() method of its own.

table.sort() will sort table in place, so you need something like this:

>>> table.sort()
>>> table.sort()

except naturally that just does the exact same sort twice in a row, which 
is silly. So what you actually need is something like this:

>>> table.sort(magic goes here)
>>> table.sort(different magic goes here)

where the first piece of magic tells Python to sort by the first column, 
and the second by the second column. But to do that, we need to know more 
about how you're setting up the table.

I'm *guessing* that you probably have something like this:


table = [ ['fred', 35, 8],  # name, age, score
    ['bill', 29, 8], 
    ['betty', 30, 9],
    ['morris', 17, 4], 
    ['catherine', 23, 6], 
    ['anna', 45, 8], 
    ['george', 19, 5],
    ['tanya', 27, 7],
    ]


Now let's sort it:


>>> from operator import itemgetter
>>> import pprint
>>> table.sort(key=itemgetter(0))  # sort by name
>>> table.sort(key=itemgetter(2))  # sort by score
>>> pprint.pprint(table)
[['morris', 17, 4],
 ['george', 19, 5],
 ['catherine', 23, 6],
 ['tanya', 27, 7],
 ['anna', 45, 8],
 ['bill', 29, 8],
 ['fred', 35, 8],
 ['betty', 30, 9]]



Does this help?



-- 
Steven