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classification
Title: Type: Sort documentation could be improved for complex sorting enhancement resolved Documentation Python 3.7
process
Status: Resolution: closed not a bug rhettinger FabriceSalvaire, SilentGhost, cheryl.sabella, docs@python, rhettinger normal

Created on 2019-02-10 16:33 by FabriceSalvaire, last changed 2022-04-11 14:59 by admin. This issue is now closed.

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) * 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) * Date: 2019-02-10 18:03
`Take a look at issue 35020 which discusses multisort.`
msg335187 - (view) Author: Raymond Hettinger (rhettinger) * 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```
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