On Wed, Sep 15, 2021 at 3:54 PM Ethan Furman <[email protected]> wrote:
> Guido:
> > It's still an iterator, since it duck-types in most cases where an
> iterator
> > is required (notably "for", which is the primary use case for the
> iteration
> > protocols -- it's in the first sentence of PEP 234's abstract).
>
> D'Aprano:
> > I don't think it duck-types as an iterator. Here's an example:
> >
> > class A:
> > def __init__(self): self.items = [1, 2, 3]
> > def __next__(self):
> > try: return self.items.pop()
> > except IndexError: raise StopIteration
> >
> > >>> for item in A(): pass
> > ...
> > Traceback (most recent call last):
> > File "<stdin>", line 1, in <module>
> > TypeError: 'A' object is not iterable
>
> Guido:
> > Yes, we all understand that. The reason I invoked "duck typing" is that
> as
> > long as you don't use the iterator in a situation where iter() is called
> > on it, it works fine.
>
>
> I'm confused.
>
> - a "broken" iterator should be usable in `for`;
> - `A` is a broken iterator;
>
> but
>
> - `A()` is not usable in `for`.
>
> What am I missing?
>
Steven's class A is the kind of class a custom sequence might return from
its __iter__ method. E.g.
class S:
def __iter__(self):
return A()
Now this works:
for x in S(): ...
However this doesn't:
for x in iter(S()): ...
In Steven's view, A does not deserve to work in the former case: Because A
is a "broken" iterator, he seems to want it rejected by the iter() call
that is *implicit* in the for-loop.
Reminder about how for-loops work:
This:
for x in seq:
<body>
translates (roughly) to this:
_it = iter(seq)
while True:
try:
x = next(_it)
except StopIteration:
break
<body>
--
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*
<http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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