[_ZeD_]

witch is a little different thing.

python can sort according to a function (using the "key" parameter) or using a custom comparator function (using the "cmp" parameter)

as a side note, Python offers also a sorted[0] function, applyable to any sequence

[0] http://docs.python.org/2/library/functions.html#sorted

[orblivion]

I don't get the difference.

[_ZeD_]

They resolve differenti problems: say you have a list of complex object, and you wanna sort by an attribute, you can use

    mylist.sort(key=lambda element: element.attribute)

Elsewere, say you want to sort a non-omogeneous list, or according to a custom order (attribute1 descending, attribute2 ascending) cmp make this easy

[philh]

I think ve was saying, ve doesn't see the difference between python's `sort(key=...)` and perl6's `sort { one-arg block }`. (At any rate, I don't see that difference.)

[rmc]

Elsewere, say you want to sort a non-omogeneous list, or according to a custom order (attribute1 descending, attribute2 ascending) cmp make this easy

You can do this in python with the key function returning a list/tuple:

    mylist.sort(key=lambda element: (element.attribute1, -element.attribute2))

[_ZeD_]

I'm not sure you can do -"string" :)

[dded]

The Python .sort() method sorts in-place. The sorted() function returns a sorted copy, leaving the original untouched.

[raiph]

I realize this is drifting off topic, and I could well believe the current Rakudo implementation doesn't actually save the memory (I could believe it does, I just don't know) but P6, for any method, including sort:

    my $a = [2,1]; say $a.sort;  say $a # 1 2 2 1
    my $a = [2,1]; say $a.=sort; say $a # 1 2 1 2

This riffs off the general op= syntax:

    my $a = 1; say $a +  1; say $a # 2 1
    my $a = 1; say $a += 1; say $a # 2 2

[orblivion]

I meant I didn't understand the difference between Python's sort(key=..) and what's described here for Perl.

[_ZeD_]

what is described for perl is python's sort(cmp=...)

[orblivion]

Hmm? I thought the whole point of the article was "Most people know about sort(cmp=), which takes two parameters. But check out sort(key=), it only takes one parameter, which means the key function only has to run once per item", but for Perl.

[raiph]

That's my understanding too.

Lisp had a nice idiom that Randal Schwartz translated in to Perl 5 in 1994 which became known as the Schwartzian Transform: http://en.wikipedia.org/wiki/Schwartzian_Transform

It seems from the comments here that Python's key and cmp args are Python's equivalent to the ST. I'm surprised by the comment that Python 3 removed cmp. Perhaps that's a temporary omission.

This HN is about an advent article for Perl 6, which covers the same territory. Based on a look at the Rakudo code, it only implements a single custom sort closure (equivalent to Python's cmp) OR a single key closure (equivalent to Python's key).

[comex]

And incomprehensibly, Python 3 removed the cmp argument, leaving only unwary sort.