except clause syntax question

[Charles Yeomans]

On Jan 31, 2012, at 11:38 AM, Devin Jeanpierre wrote:

> On Tue, Jan 31, 2012 at 11:23 AM, Charles Yeomans
> <charles at declaresub.com> wrote:
>> 
>> On Jan 31, 2012, at 9:51 AM, Steven D'Aprano wrote:
>> 
>>> On Tue, 31 Jan 2012 08:57:31 -0500, Charles Yeomans wrote:
>>> 
>>>> I don't think of a tuple as a container, and I don't think it a
>>>> misunderstanding on my part to think this.
>>> 
>>> 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?
>> 
>> 
>> 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.
>> 
>> 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.
>> 
>> Nor am I the only person who sees such a distinction between tuple and list.
> 
> Perhaps it'd be useful to look at how the Python language reference
> defines containers?
> 
> Quote:
> 
> 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’s 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.
> 
> End quote.
> (Offtopic: How do I do an external block quote appropriately in an email?)
> 
> 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.
> 
> 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.
> 
> 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