Re: Suggested feature: slice syntax within tuples (or even moregenerally)?

Discussion in 'Python' started by Andrew Robinson, Feb 25, 2013.

  1. On 02/25/2013 10:28 AM, Ian Kelly wrote:
    > On Sun, Feb 24, 2013 at 6:10 PM, Andrew Robinson
    > <andrew3@xxx> wrote:
    >> I've read through the whole of the subject, and the answer is no,
    >> although I
    >> think allowing it in :):) is a *very* good idea, including as a
    >> replacement
    >> for range or xrange.
    >> s=1:2:3
    >> for i in s:
    >> for i in (1:2:3) :

    > Eww, no. I can appreciate the appeal of this syntax, but the problem
    > is that ranges and slices are only superficially similar. For one,
    > ranges require a stop value; slices do not. What should Python do
    > with this:
    > for i in :)):

    The same thing it would do with slices.

    A slice is converted to an iterator at the time __getitem__ is called;
    it in fact has methods to compute the actual start and stop, based on
    the parameters given and the size of the object it is applied to.
    Slices are, therefore, *not* in fact infinite;

    > Intuitively, it should result in an infinite loop starting at 0. But
    > ranges require a stop value for a very good reason -- it should not be
    > this easy to accidentally create an infinite for loop.

    It wouldn't, but even if it did an *effective* infinite loop is already
    easy to create with xrange:
    a = 10
    a = 1.1e12
    for i in xrange( int(a) ):

    and, besides, the same is true with other constructions of loops....

    while a: # Damn easy, if a is accidentally true!

    I can go on.... but it's rather pointless. Build a better protective
    device, and the world will find a luckier idiot for you. There isn't
    enough concrete to stop terrorists -- and not enough typing to stop bad
    programmers and pass the good ones.

    > So I would
    > advocate that this should raise an error instead. If the user really
    > wants an unlimited counting loop, let them continue to be explicit
    > about it by using itertools.count. On the other hand, this would mean
    > that the semantics of :)) would be different depending on whether the
    > slice is used as a slice or a range.

    No, it would be different depending on whether or not it was applied to
    an iterable; which is already true.

    > The next problem you run into is that the semantics of negative
    > numbers are completely different between slices and ranges. Consider
    > this code:
    > s = (-5:6)
    > for i in s:
    > print(i)
    > for i in range(6):
    > print(i)

    I don't find this difference to be necessary, nor objectionable.

    It is less inconsistent, in my view, to allow that
    ([ 1,2,3,4,5 ])[-1:2] produce [5,1,2] than an empty list;
    and ([ 1,2,3,4,5])[2:-1] does produce an empty list.

    I have been looking for actual programs that this would break for over
    two months now, and I haven't been finding any. I am willing to run any
    mainstream application you can find on test-patched python!

    > Intuitively, both loops should print the same thing. After all, one
    > is using the slice s as a range, and the other is using the very same
    > slice s as a slice of a sequence where the indices and values are the
    > same.

    YES! I like the way you think about consistency and intuition.

    > This expectation fails, however. The first loop prints the
    > integers from -5 to 5 inclusive, and the second loop only prints the
    > integers from 1 to 5 inclusive.
    > For these reasons, I disagree that allowing slices to be implicitly
    > converted to ranges or vice versa is a good idea.

    I respect your opinion and agree to simply disagree.
    Andrew Robinson, Feb 25, 2013
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