number ranges

[Colin J. Williams]

Alex Martelli wrote:
> John Zenger <john_zenger at yahoo.com> wrote:
>    ...
> 
>>>for i in 2 to 5:
>>>    print i,
>>>
>>>which would print 2 3 4 5
>>
>>This proposed syntax is also easy to understand, maybe more than "..",
>>because it uses natural language.  The only criticism is that it creates
>>a list without using [] notation.  How about [2 to 5]?
> 
> 
> Having this hypothetical notation create a list (rather than a genexp)
> would qualify as a blooper of the first magnitude. Historically, 'range'
> was introduced before anything but a list was an option, so it had to be
> grandfathered in (and doesn't use any [] either, BTW), but eventually
> (in 3.0) it will return an iterator (as will methods keys/values/items
> of dicts, and so forth).
> 
> If such notation was ever to appear in Python (and it won't, in part for
> reasons I expand on below), it would surely be '(2 to 5)' and return an
> iterator equivalent to what you could get today as iter(xrange(2,5)).
> 
> 
>>>(I'm open to arguments that it should be more Pythonic and less 
>>>mathematical, and halt at 4.)
>>
>>I am also open to such arguments but it will be tough to convince me 
>>that "x to y" should mean something different from what it means in 
>>Pascal, BASIC, and English.
> 
> 
> Andrew Koenig, in "C traps and pitfalls" (a great book), did a great job
> explaining why semi-open intervals (lower bound included, upper bound
> excluded) are vastly preferable. The common sloppy practice of including
> both bounds leads to frequent "off by one" errors, also known as
> "fencepost errors", which often occur in English too -- check out the
> origin of the error's name, e.g. at
> <http://catb.org/~esr/jargon/html/F/fencepost-error.html> .
> 
> Natural language has many ambiguities and subtle error-inducing traps
> (indeed, being able to say something which means different things to
> different people may have survival value for the crafty user of
> language, so it can be argued that these traps and ambiguities are
> adaptive;-). No reason for programming languages, or other formal
> languages, to follow natural language down this destructive path.
> 
> In any case, the point is crucial: since Python will never do something
> as abysmal as introducing a closed interval rather than a semiopen one,
> the same kind of people who now long for '2 to 5' (unlikely to have
> pondered Koenig's arguments, etc) will never be happy anyway, so it's
> much better to avoid introducing '2 to 5' at all.
> 
> 
> Alex
> 
Two questions:
1. Why not treat '2 to 5' or '(2 to 5)' as a semi-open interval?
2. How could a stride be included in this notation?

Colin W.