Alex Martelli wrote: > John Zenger <[EMAIL PROTECTED]> 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. -- http://mail.python.org/mailman/listinfo/python-list
