Question about exausted iterators

[Terry Reedy]

"Christophe" <chris.cavalaria at free.fr> wrote in message 
news:446b48e2$0$5293$626a54ce at news.free.fr...
> Is there a good reason why when you try to take an element from an
> already exausted iterator, it throws StopIteration instead of some other
> exception ?

Yes.
.
.
To distinguish the control message "I am done yielding values, as per the 
code specification (so don't bother calling me again)." from error messages 
that say "Something is wrong, I cannot yield values and give up."  In other 
words, to distinguish expected correct behavior from unexpected incorrect 
behavior.  This is essential for the normal and correct use of iterators.

> I've lost quite some times already because I was using a lot
> of iterators and I forgot that that specific function parameter was one.

I think you mean 'specific function argument' for a parameter which could 
be any iterable.

> Exemple :
Example

> >>> def f(i):
> ...     print list(i)
> ...     print list(i)
> ...
> >>> f(iter(range(2)))
> [0, 1]
> []

As per specification.
I am guessing that you want the first list() call to terminate normally and 
return a list, which requires exhausted i to raise StopIteration, while you 
want the second list() to not terminate but raise an exception, which 
requires exhausted i to raise something other than StopIteration.  Tough.

One solution is call list(i) exactly once:

def f(i):
    li = list(i)
    print li
    print li

Another is to document f as requiring that i be a non-iterator reiterable 
iterable and only pass correct arguments.

A third is to add a line like
    if iter(i) is i: raise TypeError("input appears to be iterator")

This is not quite exact since it will improperly exclude self-iterator 
reiterables (which, I believe, no builtin is) and improperly pass 
non-reiterable non-iterator iterables (at least some file objects).  But it 
might work for all your cases.

Terry Jan Reedy