[Python-ideas] PEP 479: Change StopIteration handling inside generators

[Chris Barker]

On Tue, Nov 25, 2014 at 9:59 AM, Guido van Rossum <guido at python.org> wrote:

> On Tue, Nov 25, 2014 at 9:47 AM, Chris Barker <chris.barker at noaa.gov>
> wrote:
>
>>
>> [...] I like how it support my goal of keeping the behavior of iterator
>> classes and generators consistent.
>>
>
> This is a great summary of the general confusion I am trying to clear up.
> The behavior of all types of iterators (including generators) from the
> *caller's* perspective is not in question and is not changing. It is very
> simple: you call next(it) (or it.__next__(), and it returns either the next
> value or raises StopIteration (and any other exception is, indeed, an
> exception).
>
> From the POV of *implementor*, iterator classes and generators have never
> been at all alike -- one uses *return* to produce a new value, the other
> uses *yield*. Returning without a value from a __next__() method is seen as
> producing a None value by the caller; returning from a generator is
> translated into a StopIteration which will be interpreted by the caller as
> the end of series.
>

Once you start nesting these things, the distinction between "implementor"
and "caller" gets mingled. And I think this is all about how nested
generators behave, yes?

If I am implementing a iterator of some sort (generator function or
iterator class), and I call next() inside my code, then I am both a
implementor and caller. And if I'm also writing helper functions, then I
need to know about how StopIteration will be handled, and it will be
handled a bit differently by generators and iterator classes.

But not a big deal, agreed, probably a much smaller deal that all the other
stuff you'd better understand to write this kind of code anyway.


-Chris


-- 

Christopher Barker, Ph.D.
Oceanographer

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Chris.Barker at noaa.gov
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