Arithmetic sequences in Python

[Tom Anderson]

On Tue, 16 Jan 2006, it was written:

> Tom Anderson <twic at urchin.earth.li> writes:
>
>> The natural way to implement this would be to make .. a normal 
>> operator, rather than magic, and add a __range__ special method to 
>> handle it. "a .. b" would translate to "a.__range__(b)". I note that 
>> Roman Suzi proposed this back in 2001, after PEP 204 was rejected. It's 
>> a pretty obvious implementation, after all.
>
> Interesting, but what do you do about the "unary postfix" (1 ..)
> infinite generator?

1.__range__(None)

>>> (-3,-5 ..)   -->  'infinite' generator that yield -3,-5,-7 and so on
>>
>> -1. Personally, i find the approach of specifying the first two 
>> elements *absolutely* *revolting*, and it would consistently be more 
>> awkward to use than a start/step/stop style syntax. Come on, when do 
>> you know the first two terms but not the step size?
>
> Usually you know both, but showing the first two elements makes sequence 
> more visible.  I certainly like (1,3..9) better than (1,9;2) or 
> whatever.

I have to confess that i don't have a pretty three-argument syntax to 
offer as an alternative to yours. But i'm afraid i still don't like yours. 
:)

>>> 1) "[]" means list, "()" means generator
>> Yuck. Yes, i know it's consistent with list comps and genexps, but yuck 
>> to those too!
>
> I'd be ok with getting rid of [] and just having generators or 
> xrange-like class instances.  If you want to coerce one of those to a 
> list, you'd say list((1..5)) instead of [1..5].

Sounds good. More generally, i'd be more than happy to get rid of list 
comprehensions, letting people use list(genexp) instead. That would 
obviously be a Py3k thing, though.

tom

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