[Python-ideas] Expose `itertools.count.start` and implement `itertools.count.__eq__` based on it, like `range`.

[Antony Lee]

I agree that "range(start=x)" is awkward due to the unusual argument
handling of "range", and you are also correct that I should have said "an
iterable for which iter(range(x, None, step)) behaves as count(n, step)".
I don't see an issue with len and negative indexing raising ValueError and
IndexError, respectively.  After all, a negative index on an infinite
sequence is just as undefined.
Antony


2014-06-07 12:07 GMT-07:00 Terry Reedy <tjreedy at udel.edu>:

> On 6/6/2014 9:59 PM, Neil Girdhar wrote:
>
>> That would be great.
>>
>
> What does 'that' refer too? Ram's original proposal, which you quoted, or
> Antony's counter-proposal, which you also quoted? Ambiguity is the cost of
> top-posting combined with over-quoting.
>
> Since I already explained what is wrong with Ram's proposal, I will delete
> it and assume you mean Antony's, which seems to not have arrived on my
> machine.
>
>
>  On Friday, May 16, 2014 12:16:52 AM UTC-4, Antony Lee wrote:
>>
>>     Actually, a more reasonable solution would be to have range handle
>>     keyword arguments and map "range(start=x)"  to "count(x)".
>>
>
> Having a parameter like 'start' mean two different things when passed by
> position and name is the sort of oddity we try to avoid.
>
> Since "a range object is an immutable, constant attribute, reiterable
> sequence object" (my earlier post), while count is an iterator, that does
> not literally work. So I will assume that you mean (looking ahead) 'an
> iterable sis_range such that iter(sis_range(n, None, step)) is the same as
> count(n, step)'.
>
>
>  Or,  perhaps more simply, "range(x, None)"
>>
>
> As an expression, stop=None is literally what you mean.
>
> The problem is that range is a finite sequence with a finite length and an
> indexable end. For instance, range(10)[-1] == 9. What is needed is a new
> semi_infinite_sequence base class 'SemiInfSeq' that allows (but not
> requires) infinite length: float('inf') or a new int('inf'). It would also
> have to disallow negative ints for indexing and slicing. Or perhaps a class
> factory is needed.
>
> Many infinite iterators whose items can be calculated from index n could
> be the iterator for a SIS subclass.  A geometric series and the sequence of
> squares are other examples. This could be a PyPI package.
>
> --
> Terry Jan Reedy
>
>
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