Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!gegeweb.org!de-l.enfer-du-nord.net!feeder1.enfer-du-nord.net!newsfeed.eweka.nl!eweka.nl!feeder3.eweka.nl!newsfeed.xs4all.nl!newsfeed5.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
MIME-Version: 1.0
In-Reply-To: <4E5E051B.8060002@v.loewis.de>
References: <50924476-9ada-487e-bd4b-5b77bce11d5b@gz5g2000vbb.googlegroups.com> <4E5E051B.8060002@v.loewis.de>
From: Ian Kelly <ian.g.kelly@gmail.com>
Date: Wed, 31 Aug 2011 09:44:47 -0600
Subject: Re: Handling 2.7 and 3.0 Versions of Dict
To: Python <python-list@python.org>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.611.1314805519.27778.python-list@python.org>
Lines: 59
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:12491

On Wed, Aug 31, 2011 at 3:55 AM, Martin v. Loewis <martin@v.loewis.de> wrot=
e:
> if sys.version_info < (3,):
>  def getDictValues(dict):
>      return dict.itervalues()
> else:
>  def getDictValues(dict):
>      return dict.values()

The extra level of function call indirection is unnecessary here.
Better to write it as:

if sys.version_info < (3,):
    getDictValues =3D dict.itervalues
else:
    getDictValues =3D dict.values

(which is basically what the OP was doing in the first place).

>> I noticed that hashing is a lot different in Python than it is in .NET
>> languages. .NET supports custom "equality comparers" that can override
>> a type's Equals and GetHashCode functions. This is nice when you can't
>> change the class you are hashing. That is why I am using a key
>> selector in my code, here. Is there a better way of overriding the
>> default hashing of a type without actually modifying its definition? I
>> figured a requesting a key was the easiest way.
>
> You could provide a Key class that takes a hash function and a value
> function:
>
> class Key:
> =A0def __init__(self, value, hash, eq):
> =A0 =A0self.value, self.hash, self.eq =3D value, hash, eq
> =A0def __hash__(self):
> =A0 =A0return self.hash(self.value)
> =A0def __eq__(self, other_key):
> =A0 =A0return self.eq(self.value, other_key.value)
>
> This class would then be used instead of your keySelector.

For added value, you can make it a class factory so you don't have to
specify hash and eq over and over:

def Key(keyfunc):
    class Key:
        def __init__(self, value):
            self.value =3D value
        def __hash__(self):
            return hash(keyfunc(self.value))
        def __eq__(self, other):
            return keyfunc(self) =3D=3D keyfunc(other)
    return Key

KeyTypeAlpha =3D Key(lambda x: x % 7)

items =3D set(KeyTypeAlpha(value) for value in sourceIterable)

Cheers,
Ian