Path: csiph.com!news.swapon.de!eternal-september.org!feeder.eternal-september.org!mx02.eternal-september.org!.POSTED!not-for-mail
From: Marko Rauhamaa <marko@pacujo.net>
Newsgroups: comp.lang.python
Subject: Re: Late-binding of function defaults (was Re: What is a function parameter =[] for?)
Date: Thu, 26 Nov 2015 12:54:05 +0200
Organization: A noiseless patient Spider
Lines: 30
Message-ID: <87610pf2uq.fsf@elektro.pacujo.net>
References: <CAPTjJmpwjWnF=d6mpgbKS1biVLoR4APutgyH0n9t6CJ=Kh4dCg@mail.gmail.com> <mailman.484.1447964295.16136.python-list@python.org> <87d1v5emhl.fsf@elektro.pacujo.net> <564e6a62$0$1620$c3e8da3$5496439d@news.astraweb.com> <56556DF8.1080407@rece.vub.ac.be> <mailman.56.1448440353.20593.python-list@python.org> <n349qb$qj4$1@dont-email.me> <878u5mns8z.fsf@elektro.pacujo.net> <n34dfe$8n5$1@dont-email.me> <874mganq2q.fsf@elektro.pacujo.net> <n34gor$kq8$1@dont-email.me> <56565822$0$1617$c3e8da3$5496439d@news.astraweb.com> <87oaeh2qy0.fsf@elektro.pacujo.net> <mailman.117.1448525450.20593.python-list@python.org> <87a8q1f9ni.fsf@elektro.pacujo.net> <mailman.120.1448526890.20593.python-list@python.org>
Mime-Version: 1.0
Content-Type: text/plain
Injection-Info: mx02.eternal-september.org; posting-host="b7cb1518d23ec19d482dcc9c31d30fdd"; logging-data="7031"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+G5wn8a/EHD9IdNkS/mwFw"
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)
Cancel-Lock: sha1:aUQW9EWF6yXmqOW4gfB+fJM0qdk= sha1:RaaviBqoPAr+o2ExH+aON4IDrIw=
Xref: csiph.com comp.lang.python:99550

Chris Angelico <rosuav@gmail.com>:

> On Thu, Nov 26, 2015 at 7:27 PM, Marko Rauhamaa <marko@pacujo.net> wrote:
>> I have wanted to use lists as keys, and there should be no reason to
>> allow mutable tuples. It should be enough to say that the behavior of
>> a dictionary is undefined if a key should mutate on the fly.
>
> Python defines dict-key-validity as being synonymous with hashability.
> It's up to the class author to make sure the object's hash is useful.

   >>> hash([])
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: unhashable type: 'list'

Annoying.

> Behaviour being undefined works just fine [1] in C. It's not so
> popular in Python. I don't think we want to have myriad new and
> experienced programmers trying to figure out why square brackets in
> dict keys are usually fine but occasionally not.

But there's no way to enforce that with any of the myriad new classes.

   Ye blind guides, which strain at a gnat, and swallow a camel.

But I say unto you, Swallow the gnat as well.


Marko