Path: csiph.com!eternal-september.org!feeder.eternal-september.org!mx02.eternal-september.org!.POSTED!not-for-mail
From: Jussi Piitulainen <jussi.piitulainen@helsinki.fi>
Newsgroups: comp.lang.python
Subject: Re: Suggestion: make sequence and map interfaces more similar
Date: Wed, 30 Mar 2016 14:21:43 +0300
Organization: A noiseless patient Spider
Lines: 22
Message-ID: <lf54mbous6w.fsf@ling.helsinki.fi>
References: <56FA8C71.4050306@rece.vub.ac.be> <CAPTjJmqjgnDvzS7=_YRf+T78GYz7J26FzWGfwrygYLeq2E9FeA@mail.gmail.com> <CABbU2U9m-35UqK5KqRj7Y_oLdq_dgH3_EScLb4Sf3UNB4oCgmQ@mail.gmail.com> <CAPTjJmp1x-5=gEwBB=n-kt6YeVd6OiN_BGh5tNoL7UPYNqKb8w@mail.gmail.com> <mailman.176.1459309120.28225.python-list@python.org> <56fb677f$0$11121$c3e8da3@news.astraweb.com> <lf5twjoo2w8.fsf@ling.helsinki.fi> <56fba7d3$0$1616$c3e8da3$5496439d@news.astraweb.com> <mailman.190.1459334001.28225.python-list@python.org> <lf5d1qcuu47.fsf@ling.helsinki.fi> <mailman.192.1459335430.28225.python-list@python.org>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Injection-Info: mx02.eternal-september.org; posting-host="305c68510616a2e7ac08bcd2ff1598bd"; logging-data="2690"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/5nGLsUSgaPrhyPpJ0PvkW/+F71oL7naA="
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)
Cancel-Lock: sha1:xtLQeXf1fg+cGCn90s6VUdYCRwo= sha1:qm9Y4ZE4kPzbGJePL0YpQU5NKmw=
Xref: csiph.com comp.lang.python:106051

Manolo Martínez writes:

> On 03/30/16 at 01:40pm, Jussi Piitulainen wrote:
>> Manolo Martínez writes:
>  >
>> > I think it's with your definition of surjection. Bijections are
>> > surjective, no?
>> 
>> Yes, and most many-to-one mappings are *not* surjective.
>
> Well, I don't know about most, there are uncountably many surjective
> and non-surjective many-to-one mappings :)

Ok, safer to say that some many-to-one mappings are not surjective.

I was thinking of finite sets, and not even really thinking. But even
with infinite domain and infinite codomain, there can be uncountably
many mappings without any of them being a surjection - just have the
codomain be a larger infinity.

It depends on the types. Which makes the concept not easily applicable
to Python data structures as such.