[Python-Dev] Pre-PEP: Allow Empty Subscript List Without Parentheses (original) (raw)

Noam Raphael noamraph at gmail.com
Fri Jun 16 15:52:33 CEST 2006

Hello,

It seems to me that people don't object to my proposal, but don't find it useful to them either.

The question is, what to do next. I guess one possibility is to raise this discussion again in a few months, when people will be less occupied with 2.5 beta. This is ok, although I would prefer a decision before that, because it might affect the design of the library - should I find a permanent workaround, or one that I know that will be removed in the future.

If you do want to continue the discussion to reach a decision, please do. You can say that if nobody else on python-dev is interested, it shouldn't be implemented. You can examine my use case, say if you think it's reasonable, and suggest alternative solutions - or say that you see how allowing empty subscript list solves it elegantly (yes!)

My point is, I don't want this discussion to naturally die because nobody is interested, since I am interested. So please say what you think should happen to it, so we can reach a conclusion.

Now, if a the discussion is to continue, Nick proposed an alternative:

2006/6/11, Nick Coghlan <ncoghlan at gmail.com>:

For your specific use cases, though, I'd be inclined to tweak the API a bit, and switch to using attributes for the single-valued data:

taxrates.incometax = 0.18

It's probably ok, although I would prefer not having to artificially group scalars just to make them attributes of something. I would prefer remaining with one object, and having something like income_tax.setvalue(), or even income_tax.value.

Although the income tax rate should actually depend on the current financial year, since it can change over time as the government increases taxes ;)

But that's exactly why I prefer writing simply "income_tax[] = 0.18" when it's a constant, which is completely analogous to "income_tax[2005] = 0.17; income_tax[2006] = 0.18" when it depends on something.

By the way, another thing about consistency: A friend of mine brought the point that there isn't another example of forbidden empty brackets

And about the other thing Nick said:

I guess I'm really only -0 on the idea of x[] invoking x.getitem(), and allowing the class to decide whether or not to define a default value for the subscript. I wouldn't implement it myself, but I wouldn't object strenuously if Guido decided it was OK :)

I would prefer an empty tuple, since invoking getitem with no arguments would be a special case: for all other possible subscript lists, exactly one argument is passed to getitem. This leaves us with one special case: a subscript list with one item and without a trailing comma results in getitem not getting a tuple, where in all other cases it does get a tuple. This works exactly like parentheses: they don't mean a tuple only when there's one item inside them and no trailing comma.

Good bye, Noam

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