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[Python-ideas] Pass a function as the argument "step" of range()
Nick Coghlanncoghlan at gmail.com
Sat Jul 4 02:17:10 CEST 2015
On 4 Jul 2015 7:25 am, "Terry Reedy" <tjreedy at udel.edu> wrote:>> On 7/3/2015 7:23 AM, Nick Coghlan wrote:>>>> On 3 July 2015 at 06:20, Pierre Quentel <pierre.quentel at gmail.com> wrote:>>>>>> @Steven, Mark>>> The definition of range() in Python docs says :>>>>>> Python 2.7 : "This is a versatile function to create lists containing>>> arithmetic progressions. It is most often used in for loops.">>>>>> Python 3.4 : "The range type represents an immutable sequence ofnumbers and>>> is commonly used for looping a specific number of times in for loops.">>>>>> Pierre, I *wrote* the Python 3 range docs.>>> I think deleting 'arithmetic' was a mistake. Would you mind changing'immutable sequence' to 'immutable arithmetic sequence'?Sure, clarifications aren't a problem - getting "arithmetic progression"back in the docs somewhere will be useful to folks familiar with themathematical terminology for how range works.> Also, I think 'numbers' should be narrowed to 'integers' (or whatever isactually accepted). The idea of allowing floats has been proposed at leastonce, probably more, and rejected.Unfortunately, we don't have a great word for "implements __index__", as"integer" at least arguably implies specifically "int".>> There isn't a "general idea" behind Python 3's range type, there's a>> precise, formal definition.>>> 'predictable finite increasing or decreasing arithmetic sequence ofintegers, efficiently implemented'Ah, I like that.> range() looks at the value of step to decide whether to raise or return.Something must look as the sign of step to decide whether stop is a max ormin, and what comparison to use. Since the sign is constant, thisdetermination need only be done once, though I do not know where or when itis currently done.>> Given that a function has neither a value nor sign, and each call canhave not only a different value, but a different sign. A step function isa very messy fit to an api with a stop parameter whose meaning depends onthe sign of step. For many sequences, one would want an explicit max or minor both.Yeah, I started trying to think of how to do this generically, and I thinkit would need to be considered in terms of upper and lower bounds, ratherthan a single stop value.That is, there'd be 6 characterising values for a general purpose computedsequence:* domain start* domain stop* domain step* range lower bound* range upper bound* item calculationHowever, you fundamentally can't make len(obj) an O(1) operation in thatmodel, since you don't know the output range without calling the function.So the general purpose form could be an iterator like: def within_bounds(iterable, lower, upper): for x in iterable: if not lower <= x < upper: break yield xPositive & negative infinity would likely suffice as defaults for the lower& upper bounds.Cheers,Nick.-------------- next part --------------An HTML attachment was scrubbed...URL: <http://mail.python.org/pipermail/python-ideas/attachments/20150704/86b11a28/attachment.html>
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