05-09-2009 Steven D'Aprano steve@remove-this-cybersource.com.au wrote:
On Fri, 04 Sep 2009 22:37:15 +0200, Jan Kaliszewski wrote:
Named tuples (which indeed are really very nice) are read-only, but the approach they represent could (and IMHO should) be extended to some kind of mutable objects.
[snip]
What sort of extensions did you have in mind?
Two useful (from my point of view) concepts have appeared (or been linked to) in this thread -- on python-list and python-ideas:
* the namespace/AttrDict concept (see Ken Newton's, Nick Coghlan's and my posts).
* record concept (see George Sakkis post).
The old discussion, the above link points to, shows that such a dot-accessible dict-like class is something that many people need and repeatedly implemet it (more or less perfectly) for themselves.
I think it's something which people copy from other languages because that's what they're used to, not because they need it.
I don't think so, especially if we say about the former. IMHO it is simply useful in practice, especially for scripting (but not only) -- being more convenient than using empty class.
It offers (in compact way, without additional efforts and verbose syntax -- once you have got such a tool implemented) three things at the same time, without necessity to choose between them: comfortable static attribute access, flexible dict-like dynamic access when needed and possibility of iteration.
It's just a change in syntax. Whether you write x.key or x['key'] is a matter of convenience. Attribute access is optimized for when you know the key names at compile time, key access is optimized for when you don't know the names until runtime.
Exactly. It is a matter of *convenience* (as well as large areas of Python) and that's the point. I suppose that that is the reason for people to repeatedly implement it for themselves.
05-09-2009 Steven D'Aprano steve@remove-this-cybersource.com.au wrote:
On Fri, 04 Sep 2009 22:51:39 -0700, Ken Newton wrote:
[snip]
I would think this is much more than just copy from other language styles or 'just' a syntax change -- the apparent widespread use would hint at a deeper need.
"Apparent" is the key word there. There are lots of people who *say* this this useful functionality, but how many of them *actually* use it? And of those who do use it, how many of them know what they're doing? There are an awful lot of bad programmers out there.
If you do need such functionality, it's easy to implement. Here's one:
Neither you nor me have hard evidence about popularity/unpopularity of the idea (number of places where you can find similar, more or less successful, attempts to implement it seems to testify in favour of the idea) -- nor about how it is used or abused.
Obviously there are a lot of bad programmers who are able to use globals instead of function arguments etc.... Thats the fate of every language feature.
But it's not the reason to resign from a feature that has particular common and proper use-cases. Even official Python tutorial mentions a case that is typical for the matter:
http://docs.python.org/3.1/tutorial/classes.html#odds-and-ends
As a general rule, if obj.x is an attribute, then every valid obj should have an attribute x. But if obj['x'] is a key/value, then it is data-specific: some instances will have an 'x' key, and some won't.
It's often true but not always (see e.g. the above example in docs).
Cheers, *j
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Jan Kaliszewski