PEP 276 -- What else could iter(5) mean?

[Huaiyu Zhu]

On Sun, 03 Mar 2002 20:04:44 GMT, logistix <logstx at bellatlantic.net> wrote:
[ example using the proposed 'for i in n' syntax ]
>
>When you run the script, a user is prompted for some numbers.  If they enter
>1,2,3,4,5, a tuple of (1,2,3,4,5) is created that can then be iterated.  If
>they just enter 5, then they probably only want to perform this operation on
>5, but when functionInMyModule() is called, the results will be identical to
>the first example, when you really want it to throw an error or create a
>tuple of (5).

This is indeed one of the best counter examples for this proposal.  

I do not think that there is any conceptual difficulty to explain that
        for i in n    ==>    for i in iter(n)
        iter(3)       ==>    xrange(3)

which is all that is in the proposal.  This is elegant and easily
explainable even to a newbie.  I actually like this quite a lot.

However, I still think the potential for unintended effects (bugs) is too
big when a one-element sequence behaves substantially different from the
element itself.  Imagine if Python had a character type and that
        "abc" == ['a', 'b', 'c'] == 'd' != "d"

Even though it can be explained, what utter confusion it would result in
actual use!

Here's another example of a similar nature.  In Matlab there is a fuction
sum(x) that would return sum of elements of x, if x is a (row or column)
vector, or a row vector of columnwise sums of x, if x is a matrix.  This is
very handy when used in isolation, but a pain in programs, because the
behavior on (1xm) matrix has a different semantics.  Suppose x is a (5x4)
matrix.  Then (using Python syntax instead of Matlab's)
        sum(a[0:n:, :])
would give a (nx4) matrix for n in 5, 4, 3, 2, but for n==1, it gives a
scalar which is the sum of x as a row vector.  

It is very expensive to guard against such "discontinuity of semantics" that
the cost completely overweigh the benefit.

Huaiyu