Re: [Python-ideas] Optimizing list.sort() by checking type in advance

[Greg Ewing]

Elliot Gorokhovsky wrote:
if the list is all 
floats, just copy all the floats into a seperate array, use the standard 
library quicksort, and then construct a sorted PyObject* array.


My question would be whether sorting list of just floats
(or where the keys are just floats) is common enough to
be worth doing this.

--
Greg
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Re: [Python-ideas] Optimizing list.sort() by checking type in advance

[Jonathan Goble]

On Mon, Oct 10, 2016 at 11:30 PM Elliot Gorokhovsky <
elliot.gorokhov...@gmail.com> wrote:

> - I expect tuples will also be worth specializing (complex sort keys are
> often implemented as tuples).
>
> I'm not sure what you mean here... I'm looking at the types of lo.keys,
> not of saved_ob_item (I think I said that earlier in this thread by mistake
> actually). So if someone is passing tuples and using itemgetter to extract
> ints or strings or whatever, the current code will work fine; lo.keys will
> be scalar types. Unless I misunderstand you here. I mean, when would
> lo.keys actually be tuples?
>

If someone wanted to sort, e.g., a table (likely a list of tuples) by
multiple columns at once, they might pass the key function as
`itemgetter(3, 4, 5)`, meaning to sort by "column" (actually item) 3, then
columns 4 and then 5 as tiebreakers. This itemgetter will return a new
tuple of three items, that tuple being the key to sort by. Since tuples
sort by the first different item, in this theoretical example the result of
sort() will be exactly what the user wanted: a table sorted by three
columns at once.

A practical example of such a use case is sorting by last name first and
then by first name where two people have the same last name. Assuming a
list of dicts in this case, the key function passed to sort() would simply
be `itemgetter('lastname", "firstname")`, which returns a tuple of two
items to use as the key.

So yes, there are perfectly valid use cases for tuples as keys.
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Re: [Python-ideas] Optimizing list.sort() by checking type in advance

[Elliot Gorokhovsky]

Oh no, the idea here is just you would copy over the floats associated with
the PyObject* and keep them in an array of such structs, so that we know
which PyObject* are associated with which floats. Then after the standard
library quicksort sorts them you would copy the PyObject* into the list. So
you sort the PyObject* keyed by the floats. Anyway, I think the copying
back and forth would probably be too expensive, it's just an idea. Also, I
apologize for the formatting of my last email, I didn't realize Inbox would
mess up the quoting like that. I'll ensure I use plain-text quotes from now
on.

On Mon, Oct 10, 2016 at 9:38 PM Chris Angelico  wrote:

> On Tue, Oct 11, 2016 at 2:29 PM, Elliot Gorokhovsky
>  wrote:
> > Ya, I think this may be a good approach for floats: if the list is all
> > floats, just copy all the floats into a seperate array, use the standard
> > library quicksort, and then construct a sorted PyObject* array. Like
> maybe
> > set up a struct { PyObject* payload, float key } type of deal.
>
> Not quite sure what you mean here. What is payload, what is key? Are
> you implying that the original float objects could be destroyed and
> replaced with others of equal value? Python (unlike insurance claims)
> guarantees that you get back the exact same object as you started
> with.
>
> ChrisA
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Re: [Python-ideas] Optimizing list.sort() by checking type in advance

[Chris Angelico]

On Tue, Oct 11, 2016 at 2:29 PM, Elliot Gorokhovsky
 wrote:
> Ya, I think this may be a good approach for floats: if the list is all
> floats, just copy all the floats into a seperate array, use the standard
> library quicksort, and then construct a sorted PyObject* array. Like maybe
> set up a struct { PyObject* payload, float key } type of deal.

Not quite sure what you mean here. What is payload, what is key? Are
you implying that the original float objects could be destroyed and
replaced with others of equal value? Python (unlike insurance claims)
guarantees that you get back the exact same object as you started
with.

ChrisA
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