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Subject: Re: except clause syntax question
From: Charles Yeomans <charles@declaresub.com>
In-Reply-To: <CABicbJ+0it37UPXaOinBvp+y46TWBsatHZgoFJEHy3Ehh7YisQ@mail.gmail.com>
Date: Tue, 31 Jan 2012 12:29:43 -0500
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On Jan 31, 2012, at 11:38 AM, Devin Jeanpierre wrote:

> On Tue, Jan 31, 2012 at 11:23 AM, Charles Yeomans
> <charles@declaresub.com> wrote:
>>=20
>> On Jan 31, 2012, at 9:51 AM, Steven D'Aprano wrote:
>>=20
>>> On Tue, 31 Jan 2012 08:57:31 -0500, Charles Yeomans wrote:
>>>=20
>>>> I don't think of a tuple as a container, and I don't think it a
>>>> misunderstanding on my part to think this.
>>>=20
>>> Well, it is a misunderstanding, because tuples ARE containers. You =
might
>>> as well say "I don't think of boxes as containers". What exactly are =
they
>>> if not containers?
>>=20
>>=20
>> Tuple is a heterogenous datatype that allows one to define objects ad =
hoc. That is to say, a tuple represents a single thing distinct from its =
components.  For example, suppose you need to represent a location in =
text by line number and offset within a line.  A tuple object makes it =
easy to do so without writing a class having no methods other than a =
constructor.  Here, the components, a line number and an offset, define =
a new object distinct from the pieces.
>>=20
>> One can certainly view a tuple as a list, just as one can view a =
string as a list of characters, and sometimes that's useful; the Python =
dictum "there should only be one way to do it" doesn't imply that there =
is only one way to think of it.
>>=20
>> Nor am I the only person who sees such a distinction between tuple =
and list.
>=20
> Perhaps it'd be useful to look at how the Python language reference
> defines containers?
>=20
> Quote:
>=20
> Some objects contain references to other objects; these are called
> containers. Examples of containers are tuples, lists and dictionaries.
> The references are part of a container=92s value. In most cases, when =
we
> talk about the value of a container, we imply the values, not the
> identities of the contained objects; however, when we talk about the
> mutability of a container, only the identities of the immediately
> contained objects are implied. So, if an immutable container (like a
> tuple) contains a reference to a mutable object, its value changes if
> that mutable object is changed.
>=20
> End quote.
> (Offtopic: How do I do an external block quote appropriately in an =
email?)
>=20
> Tuples are most certainly containers, precisely _because_ they're an
> ad-hoc way to define objects, where the only purpose of the object is
> to contain the values inside the tuple.
>=20
> But these are just words and it's beside the point. We should talk of
> things as if the word "container" didn't matter, because I don't think
> that's what you meant, but neither do I want to put words in your
> mouth.
>=20
> My interpretation is that you see tuples as an object where you can
> get meaningfully things by field, rather than just grab arbitrarily
> from a bag of things. This isn't the only way they are used, see the
> except statement (hee) and their use as keys in dictionaries. But it
> is true that their immutable length makes them very well suited to the
> task of representing product types.

I had read that bit of documentation, and don't entirely agree with it.  =
Certainly many objects contain references to other objects, but are not =
considered containers.

And I claim that tuples are most certainly not containers, when they are =
used to define an object as a collection of other objects, like the =
text-location example I offered earlier.  On the other hand, it is =
certainly true that tuples quack like containers, as do strings.


Charles Yeomans