On Tue, Sep 14, 2021 at 09:38:38PM -0700, Guido van Rossum wrote:
> > I don't know what I would call an object that only has __next__,
> > apart from "broken" :-(
> >
>
> 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).
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
class B:
def __iter__(self):
return A()
It's fine to iterate over B() directly, but you can't iterate over
A() at all. If you try, you get a TypeError:
>>> for item in A(): pass
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'A' object is not iterable
In practice, this impacts some very common techniques. For instance,
pre-calling iter() on your input.
>>> x = B()
>>> it = iter(x)
>>> for value in it: pass
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'A' object is not iterable
There are all sorts of reasons why one might pre-call iter(). One common
one is to pre-process the first element:
it = iter(obj)
first = next(obj, None)
for item in it: ...
Another is to test for an iterable. iter(obj) will raise TypeError if
obj is not a sequence, collection, iterator, iterable etc.
Another is to break out of one loop and then run another:
it = iter(obj)
for x in it:
if condition: break
do_something()
for x in it:
something_else()
I'm sure there are others I haven't thought of.
I believe that iterable objects that define `__next__` but not
`__iter__` are fundamentally broken. If they happen to work in some
circumstances but not others, that's because the iterator protocol is
relaxed enough to work with broken iterators :-)
--
Steve
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