[Python-ideas] Introduce collections.Reiterable

Neil Girdhar mistersheik at gmail.com
Sat Sep 21 23:14:56 CEST 2013

On Sat, Sep 21, 2013 at 5:08 PM, Andrew Barnert <abarnert at yahoo.com> wrote:

> On Sep 21, 2013, at 0:21, Neil Girdhar <mistersheik at gmail.com> wrote:
> I'm happy with iterable and not iterator if it comes with a promise.  Then
> my first ABC is what what I probably want.  If not, then I think it's
> better to do something lke
> class Reiterable(collections.Iterable,
>                  metaclass=collections.abc.ABCMeta):
>     @classmethod
>     def __subclasshook__(cls, subclass):
>         if (issubclass(subclass, collections.MappingView)
>             or issubclass(subclass, collections.Sequence)
>             or issubclass(subclass, collections.Set)
>             or issubclass(subclass, collections.Mapping)):
>             return True
>         return NotImplemented
> Which leaves out numpy arrays, most sorted list and dict classes from
> PyPI, ElementTree and similar element/node/etc. types,
> ScriptingBridge/appscript collections, win32com IWhateverCollections, and
> all kinds of other types that can be reiterated, which are correctly
> diagnosed by Iterable and not Iterator.
> I haven't tested all of them, so some could fail to register as Iterable
> (especially given the possibility that Iterable may be incorrect, as
> mentioned elsewhere on this thread). But getting false negatives on a few
> types and having to deal with them by fixing a bug is surely better than
> getting false negatives on all types and having to deal with them by adding
> new, otherwise-unnecessary code.
> Other classes can be added with register.
> So anyone who wants to use your module with numpy or appscript or
> ElementTree has to find all of the iterable types the class exposes (some
> of which aren't part of the public API--in some the case of appscript or
> win32com the may even be built dynamically as needed) and register all of
> them?
> You're putting the burden in the wrong place. Because you're worried that
> some class could theoretically be a non-reiterable non-iterator iterable,
> even though neither you nor anyone else can think of a sensible example of
> such a thing, you're requiring the user to certify that every iterable
> single class he uses is not pathological. That's not LBYL, that's perform a
> comprehensive survey and environmental impact report on the entire region
> and file papers in triplicate before you leap.

If you really think that there will never be a non-reiterable non-iterator
iterable, then the standard should promise that and we're in total

> If you're really worried about this unlikely possibility making it hard to
> debug the use of your code with some as-yet-unknown type, there are easier
> ways to verify things. For example, if the iterable works the first time,
> but is empty the second, the user has given you a non-reiterable, and you
> can assert or raise appropriately, which will make the code error just as
> easy to debug as having forgotten to register with Reiterable--and far
> easier to debug than having mistakenly registered with Reiterable when they
> shouldn't have. Plus, this lets you test for exactly what you want, not
> just a rough approximation. You could just as easily verify that the first
> element of each iteration matches, to ensure that it's not a
> random-reiterable type like Terry discussed that would ruin your particular
> two-pass algorithm. Or whatever is appropriate.

I think "asserting on" the iterator that was passed in is a much worse
solution than "casting it to iterable".  Don't annoy the user with
implementation details is a good rule to follow.



> On Sat, Sep 21, 2013 at 3:04 AM, Andrew Barnert <abarnert at yahoo.com>wrote:
>> On Sep 20, 2013, at 21:52, Neil Girdhar <mistersheik at gmail.com> wrote:
>> We discussed this upthread: I only want "not iterator" if not iterator
>> promises reiterability. Right now, we have what may be a happy accident
>> that can easily be violated by someone else.
>> And if you define your new ABC, it can be just as easily violated by
>> someone else. In fact, it will be violated in the exact _same_
>> cases. There's no check you can do besides the reverse of the checks done
>> by iterator.
>> More importantly, it's not just "a happy accident". I've asked repeatedly
>> if anyone can come up with a single example of a non-iterator,
>> non-reiterable iterator, or even imagine what one would look like, and
>> nobody's come up with one. And it's not like iterators are some new feature
>> nobody's had time to explore yet.
>> So, in order to solve a problem that doesn't exist, you want to add a new
>> feature that wouldn't solve it any better than what we have today.
>>  Best,
>> Neil
>> On Sat, Sep 21, 2013 at 12:50 AM, Andrew Barnert <abarnert at yahoo.com>wrote:
>>> On Sep 20, 2013, at 21:23, Neil Girdhar <mistersheik at gmail.com> wrote:
>>> I appreciate the discussion illuminating various aspects of this I
>>> hadn't considered. Finally, what I think I want is for
>>> * all sequences
>>> * all views
>>> * numpy arrays
>>> to answer yes to reiterable, and
>>> * all generators
>>> to answer no to reiterable.
>>> All sequences, views, and numpy arrays answer no to iterator (and so do
>>> sets, mappings, etc.), and all generators answer yes (and so do the
>>> iterators you get back from calling iter on a sequence, map, filter, your
>>> favorite itertools function, etc.)
>>> So you just want "not iterator". Even Haskell doesn't attempt to provide
>>> negative types like that. (And you can very easily show that it's iterator
>>> that's the normal type: it's syntactically checkable in various ways--e.g.,
>>> it.hasattr('__next__'), but the only positive way to check reiterable is
>>> not just semantic, but destructive.)
>>> Best, Neil
>>> On Fri, Sep 20, 2013 at 10:12 PM, Stephen J. Turnbull <
>>> stephen at xemacs.org> wrote:
>>>> Terry Reedy writes:
>>>>  > Dismissing legal code as 'pathological', as more than one person has,
>>>>  > does not cut it as a design principle.
>>>> But you don't even need to write a class with __getitem__() to get
>>>> that behavior.
>>>> >>> l = [11, 12, 13]
>>>> >>> for i in l:
>>>> ...  print(i)
>>>> ...  if i%2 == 0:
>>>> ...   l.remove(i)
>>>> ...
>>>> 11
>>>> 12
>>>> >>> l
>>>> [11, 13]
>>>> >>>
>>>> Of course the iteration itself is probably buggy (ie, the writer
>>>> didn't mean to skip printing '13'), but in general iterables can
>>>> change themselves.
>>>> Neil himself seems to be of two minds about such cases.  On the one
>>>> hand, he said the above behavior is built in to list, so it's
>>>> acceptable to him.  (I think that's inconsistent: I would say the
>>>> property of being completely consumed is built in to iterator, so it
>>>> should be acceptable, too.)  On the other hand, he's defined a
>>>> reiterable as a collection that when iterated produces the same
>>>> objects in the same order.
>>>> Maybe what we really want is for copy.deepcopy to do the right thing
>>>> with iterables.  Then code that doesn't want to consume consumable
>>>> iterables can do a deepcopy (including replication of the closed-over
>>>> state of __next__() for iterators) before iterating.
>>>> Or perhaps the right thing is a copy.itercopy that creates a new
>>>> composite object as a shallow copy of everything except that it clones
>>>> the state of __next__() in case the object was an iterator to start
>>>> with.
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