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

[Nick Coghlan]

On 20 November 2014 06:48, MRAB <python at mrabarnett.plus.com> wrote:

> On 2014-11-19 20:10, Guido van Rossum wrote:
>
>> There's a new PEP proposing to change how to treat StopIteration
>> bubbling up out of a generator frame (not caused by a return from
>> the frame). The proposal is to replace such a StopIteration with a
>> RuntimeError (chained to the original StopIteration), so that only
>> *returning* from a generator (or falling off the end) causes the
>> iteration to terminate.
>>
>>  The PEP says """any generator that depends on an implicitly-raised
> StopIteration to terminate it will have to be rewritten to either catch
> that exception or use a for-loop"""
>
> Shouldn't that be "... explicitly-raised ...", because returning raises
> StopIteration implicitly? ("raise StopIteration" is explicit)


The ways a generator can currently be terminated:

    return [value]
    raise StopIteration[([value])]
    <any other expression that may raise StopIteration>

The first case is unchanged, and (as Chris noted), the second case can be
trivially converted to the first case.

It's the third implicit case which would no longer be allowed, which means
no more "or stop()" tricks to get generator expressions to terminate early.
(If folks wanted to restore that capability, they'd instead need to propose
new syntax that works the same way in both comprehensions and generator
expressions, rather than relying on the lazy evaluation of generator
expressions)

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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