classification
Title: Sort documentation could be improved for complex sorting
Type: enhancement Stage:
Components: Documentation Versions: Python 3.7
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: rhettinger Nosy List: FabriceSalvaire, SilentGhost, cheryl.sabella, docs@python, rhettinger
Priority: normal Keywords:

Created on 2019-02-10 16:33 by FabriceSalvaire, last changed 2019-02-11 01:39 by rhettinger.

Messages (4)
msg335163 - (view) Author: fabrice salvaire (FabriceSalvaire) Date: 2019-02-10 16:33
I just implemented Graham Scan algorithm in Python 3 and have to read carefully the sort documentation.  Notice this is a good algorithm for a large audience language like Python.

Since Python 3, the old order function cmp is depicted as an old way to proceed.

But some sorting procedure require complex order like this

    def sort_by_y(p0, p1):
        return p0.x - p1.x if (p0.y == p1.y) else p0.y - p1.y
    sorted(points, key=cmp_to_key(sort_by_y))

which is less natural to implement than

    def sort_by_y(p0, p1):
        return p0.x < p1.x if (p0.y == p1.y) else p0.y < p1.y
    sorted(points, cmp=sort_by_y)

Since Python 3 we should do this

    points.sort(key=attrgetter('x'))
    points.sort(key=attrgetter('y'))

But we must take care to the chaining order !!! Here we must sort first on x then on y.

I think the documentation could explain much better how to perform complex sort and the performance of the Python sort algorithm.  Is the old way faster than the new one ???  What about short and large array ???  What happen when we sort a zillion of short array ???
msg335164 - (view) Author: SilentGhost (SilentGhost) * (Python triager) Date: 2019-02-10 16:37
Is this not equivalent to the following?

  sorted(points, key=lambda p: (p.y, p.x))
msg335167 - (view) Author: Cheryl Sabella (cheryl.sabella) * (Python triager) Date: 2019-02-10 18:03
Take a look at issue 35020 which discusses multisort.
msg335187 - (view) Author: Raymond Hettinger (rhettinger) * (Python committer) Date: 2019-02-11 01:39
It seems to me that the sorting howto covers this topic.

If I'm reading the OP's task correctly, it isn't complex at all:

   points.sort(key=attrgetter('x', 'y'))  # x is primary key; y is secondary
History
Date User Action Args
2019-02-11 01:39:51rhettingersetassignee: docs@python -> rhettinger

messages: + msg335187
nosy: + rhettinger
2019-02-10 18:03:40cheryl.sabellasetnosy: + cheryl.sabella
messages: + msg335167
2019-02-10 16:37:44SilentGhostsetnosy: + SilentGhost
messages: + msg335164
2019-02-10 16:33:39FabriceSalvairecreate