Re: Sorting a List of Lists

On Jan 31, 12:35 pm, Bruno Desthuilliers <bruno.
42.desthuilli...@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:
apoth...@xxxxxxxxx a écrit :

I can't seem to get this nailed down and I thought I'd toss it out
there as, by gosh, its got to be something simple I'm missing.

I have two different database tables of events that use different
schemas. I am using python to collate these records for display. I do
this by creating a list of lists that look roughly like this:

events = [['Event URL as String', 'Event Title as String ', Event Date
as Datetime], ...]

Then you should not use a list of lists, but a list of tuples.



I then thought I'd just go events.sort(lambda x,y: x[2]<y[2]) and call
it a day. That didn't work. But then lamda functions like to be very
simple, maybe object subscripts aren't allowed (even though I didn't
get an error). So I wrote a comparison function that looks much as you
would expect:

def date_compare(list1,
list2):
x = list1[2]
y = list2[2]
if
x>y:
return
1
elif
x==y:
return
0
else: #
x<y
return -1

But as before sorting with this function returns None.

What have I overlooked?

Lol.

I guess this is a FAQ. list.sort() performs a destructive in-place sort,
and always return None. This is in the FineManual:

bruno@bruno:~$ python
Python 2.4.4c1 (#2, Oct 11 2006, 21:51:02)
[GCC 4.1.2 20060928 (prerelease) (Ubuntu 4.1.1-13ubuntu5)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> help(list.sort)
Help on method_descriptor:

sort(...)
L.sort(cmp=None, key=None, reverse=False) -- stable sort *IN PLACE*;
cmp(x, y) -> -1, 0, 1

You may want to use sorted(iterable, cmp=None, key=None, reverse=False)
if you don't want to sort in-place.

Also, using comparison functions is usually not the most efficient way
to do such a sort. In your case, I'd go for a good old
Decorate/sort/undecorate (AKA schwarzian transform):

events = [evt for date, evt in
sorted([(evt[2], evt) for evt in events])]

HTH

I agree with you B., but see the comments here:
http://www.biais.org/blog/index.php/2007/01/28/23-python-sorting-
efficiency
for information on the relative speeds of rolling your own DSU versus
using itemgetter and key=...

- Paddy.


.

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