Is there a way to sort custom objects in a custom way in Python?

Question

I'm using web.py to spin up endpoints and as such, I present a list of endpoints like so

urls = ('/(.*)', 'base', 
        '/shapes/(.*)', 'shapes', 
        '/sounds/(.*)', 'sounds', 
        '/shapes/rectangualar/(.*)', 'rectangualarShapes', 
        '/sounds/loud/(.*)', 'loudSounds')

As web.py will see the first one and match all possible endpoints to it, they need to be ordered, most specific first, i.e.

urls = ('/shapes/rectangualar/(.*)', 'rectangualarShapes', 
        '/shapes/(.*)', 'shapes',
        '/sounds/loud/(.*)', 'loudSounds' 
        '/shapes/(.*)', 'shapes', 
        '/sounds/(.*)', 'sounds',
        '/(.*)', 'base')

I want to order these, by URI. Firstly if there's a simple way to do it, as tuples, it would be great but I figure I need to convert them to objects

class Endpoint(object):
    def __init__(self, id, classname):
        self.id = id
        self.classname = classname

    def getId(item):
        return self.id

and print sorted(endpointList, key=id) them. How would I do this in Python 3+? I've come across comparator functions but it seems they are deprecated.

What is the best way to do custom sorting?

Alex Hall · Accepted Answer · 2016-05-20 09:49:02Z

2

sorted(endpointList, key=lambda endpoint: endpoint.id)

or

sorted(endpointList, key=Endpoint.getId)

id is a builtin function that does something else which you don't want, hence your code not working.

answered May 20, 2016 at 9:49

Alex Hall

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4 Comments

jfs Over a year ago

there is also key=operator.attrgetter('id', 'classname')

Ross Drew Over a year ago

This doesn't work, result:-

[/(.*) -> 'base', /shapes/(.*) -> 'shapes', /shapes/rectangualar/(.*) -> 'rectangualarShapes', /sounds/(.*) -> 'sounds', /sounds/loud/(.*) -> 'loudSounds']

The less specific /(.*) url is still at the top of the list. Isn't sort just trying to do it alphabetically?

Alex Hall Over a year ago

Sorry, I just saw key=id and jumped to that as a likely problem. Yes, it's alphabetical. I don't know how to order by specificity. But key functions are indeed the usual way to sort a list. If you can really only think of how to express this using a comparator function, check out functools.cmp_to_key.

Ross Drew Over a year ago

the solution by @Ilja Everilä actually works well :)

Ross Drew · Accepted Answer · 2016-05-20 10:44:38Z

1

Firstly if there's a simple way to do it, as tuples it would be great

Without wrapping in a custom object:

from itertools import chain
from operator import itemgetter

sorted_urls = tuple(chain.from_iterable(sorted(zip(urls[::2],
                                                   urls[1::2]),
                                               key=itemgetter(0), reverse=True)))

If using chain is not your thing, flatten with generator expressions:

tuple(x for pair in sorted(zip(urls[::2], urls[1::2]),
                           key=itemgetter(0), reverse=True)
      for x in pair)

edited May 20, 2016 at 10:44

Ross Drew

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answered May 20, 2016 at 9:55

Ilja Everilä

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5 Comments

Ross Drew Over a year ago

Didn't try the first one but the second one works a treat. Now I need to get my head around how it works :P

Ilja Everilä Over a year ago

It builds 2-tuples (path, name) with the zip() by stepping in increments of 2 from index 0 and 1, reverse sorts them according to path with itemgetter(0) (change the key as necessary) and flattens the result back to a tuple. The for ... for ... syntax for flattening is a bit nasty, as the innermost for is actually farthest away from the result expression.

Ross Drew Over a year ago

What really confused me was how it manages to tell the difference between fine and coarse grained URLs and didn't just attempt an alphabetical sort.

Ilja Everilä Over a year ago

Well it is just an alphabetical sort, just reversed. You could also sort with something like lambda pair: (len(pair[0]), pair[0]), which would sort them from longest to shortest, alphabetically, if reversed. All in all I guess the best thing would be to just have the configuration in the correct order to begin with :P

Ross Drew Over a year ago

Really? That's strange because it seems to sort as expected by granularity. I'll need to make some more advanced tests. Sadly, my code has no way of knowing all tuple entries up front, as you can see... github.com/rossdrew/endpointSim.py

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