On Tue, Sep 14, 2021 at 11:44 PM Steven D'Aprano <steve@pearwood.info> wrote:
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
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. Just like a class with a readline() method works fine in some cases where a file is expected.
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.
No-one is arguing that an iterator that doesn't define __iter__ is great. And the docs should continue to recommend strongly to add an __iter__ method returning self. My only beef is with over-zealous people who might preemptively want to reject an iterator at runtime that only has __next__; in particular "for" and iter() have no business checking for this attribute ("for" only needs __next__, and iter() only should check for the minimal version of the protocol to reject things without __next__).
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 :-)
Your opinion is loud and clear. I just happen to disagree. -- --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-c...>