Re: What use is 'reduce'?

Path	csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From	Peter Otten <__peter__@web.de>
Newsgroups	comp.lang.python
Subject	Re: What use is '__reduce__'?
Date	Fri, 08 Jan 2016 21:12:18 +0100
Organization	None
Lines	32
Message-ID	<mailman.79.1452283950.2305.python-list@python.org> (permalink)
References	<399e214a-552b-4d4a-a39f-8e917dadd6c6@googlegroups.com> <CALwzidk-R-VVVi+59hRODxceb_rBr5WS=v+p8hOyLR1sZNPP8A@mail.gmail.com>
Mime-Version	1.0
Content-Type	text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding	7Bit
X-Trace	news.uni-berlin.de nXsXTS0j9dWnrIBmvMsx/Qen3A4DAN8PfLPzv11BAjPw==
Return-Path	<python-python-list@m.gmane.org>
X-Original-To	python-list@python.org
Delivered-To	python-list@mail.python.org
X-Spam-Status	OK 0.000
X-Spam-Evidence	'H': 1.00; 'S': 0.00; '5),': 0.09; 'method:': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'def': 0.13; 'missed': 0.15; "'%s(%r)'": 0.16; "('b',": 0.16; "('c',": 0.16; "('r',": 0.16; '1),': 0.16; '2),': 0.16; 'copied.': 0.16; 'received:80.91.229.3': 0.16; 'received:dip0.t-ipconnect.de': 0.16; 'received:io': 0.16; 'received:plane.gmane.org': 0.16; 'received:psf.io': 0.16; 'received:t-ipconnect.de': 0.16; 'remembers': 0.16; 'wrote:': 0.16; 'example.': 0.18; '>>>': 0.20; 'elements': 0.23; 'import': 0.24; 'header:User-Agent:1': 0.26; "doesn't": 0.26; 'example': 0.26; 'header:X-Complaints-To:1': 0.26; 'skip:( 20': 0.28; 'code': 0.30; 'probably': 0.31; 'included': 0.32; 'skip:_ 10': 0.32; 'class': 0.33; 'instances': 0.33; 'subject:use': 0.35; 'to:addr:python-list': 0.36; 'subject:: ': 0.37; 'really': 0.37; 'received:org': 0.37; "won't": 0.38; 'copying': 0.38; 'skip:o 20': 0.38; 'to:addr:python.org': 0.40; 'received:de': 0.40; 'determine': 0.61
X-Injected-Via-Gmane	http://gmane.org/
X-Gmane-NNTP-Posting-Host	p57bd972e.dip0.t-ipconnect.de
User-Agent	KNode/4.13.3
X-BeenThere	python-list@python.org
X-Mailman-Version	2.1.20+
Precedence	list
List-Id	General discussion list for the Python programming language <python-list.python.org>
List-Unsubscribe	<https://mail.python.org/mailman/options/python-list>, <mailto:python-list-request@python.org?subject=unsubscribe>
List-Archive	<http://mail.python.org/pipermail/python-list/>
List-Post	<mailto:python-list@python.org>
List-Help	<mailto:python-list-request@python.org?subject=help>
List-Subscribe	<https://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Xref	csiph.com comp.lang.python:101396

Show key headers only | View raw

Ian Kelly wrote:

> As you found by searching, __reduce__ is used to determine how
> instances of the class are pickled. If the example you're using
> doesn't do any pickling, then it's not really relevant to the example.
> It's probably included so that it won't be missed when the code is
> copied.

__reduce__() also makes copying work as expected:

>>> class OrderedCounter(Counter, OrderedDict):
...      'Counter that remembers the order elements are first seen'
...      def __repr__(self):
...          return '%s(%r)' % (self.__class__.__name__,
...                             OrderedDict(self))
...      def __reduce__(self):
...          return self.__class__, (OrderedDict(self),)
... 
>>> oc = OrderedCounter('abracadabra') 
>>> import copy
>>> copy.copy(oc)
OrderedCounter(OrderedDict([('a', 5), ('b', 2), ('r', 2), ('c', 1), ('d', 
1)]))

Now take away the __reduce__ method:

>>> del OrderedCounter.__reduce__
>>> copy.copy(oc)
OrderedCounter(OrderedDict([('b', 2), ('a', 5), ('c', 1), ('r', 2), ('d', 
1)]))

Back to comp.lang.python | Previous | Next — Previous in thread | Find similar | Unroll thread

Thread

What use is '__reduce__'? Robert <rxjwg98@gmail.com> - 2016-01-08 08:42 -0800
  Re: What use is '__reduce__'? Ian Kelly <ian.g.kelly@gmail.com> - 2016-01-08 10:03 -0700
    Re: What use is '__reduce__'? Robert <rxjwg98@gmail.com> - 2016-01-08 09:45 -0800
  Re: What use is '__reduce__'? Peter Otten <__peter__@web.de> - 2016-01-08 21:12 +0100

csiph-web