[Python-Dev] Please reconsider PEP 479.

[Nick Coghlan]

On 27 November 2014 at 11:15, Guido van Rossum <guido at python.org> wrote:
> On Wed, Nov 26, 2014 at 2:53 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>>
>> On 27 Nov 2014 06:35, "Guido van Rossum" <guido at python.org> wrote:
>>
>> [...]
>>
>> > I think we can put a number to "much faster" now -- 150 nsec per
>> > try/except.
>> >
>> > I have serious misgivings about that decorator though -- I'm not sure
>> > how viable it is to pass a flag from the function object to the execution
>> > (which takes the code object, which is immutable) and how other Python
>> > implementations would do that. But I'm sure it can be done through sheer
>> > willpower. I'd call it the @hettinger decorator in honor of the PEP's most
>> > eloquent detractor. :-)
>>
>> I agree with everything you wrote in your reply, so I'll just elaborate a
>> bit on my proposed implementation for the decorator idea.
>
> This remark is  ambiguous -- how strongly do you feel that this decorator
> should be provided? (If so, it should be in the PEP.)

I think it makes sense to standardise it, but something like
"itertools.allow_implicit_stop" would probably be better than having
it as a builtin. (The only reason I suggested a builtin initially is
because putting it in itertools didn't occur to me until later)

Including the decorator provides a straightforward way to immediately
start writing forward compatible code that's explicit about the fact
it relies on the current StopIteration handling, without being
excessively noisy relative to the status quo:

    # In a module with a generator that relies on the current behaviour
    from itertools import allow_implicit_stop

    @allow_implicit_stop
    def my_generator():
        ...
        yield next(itr)
        ...

In terms of code diffs to ensure forward compatibility, it's 1 import
statement per affected module, and 1 decorator line per affected
generator, rather than at least 3 lines (for try/except/return) plus
indentation changes for each affected generator. That's a useful
benefit when it comes to minimising the impact on version control code
annotation, etc.

If compatibility with older Python versions is needed, then you could
put something like the following in a compatibility module:

    try:
        from itertools import allow_implicit_stop
    except ImportError:
        # Allowing implicit stops is the default in older versions
        def allow_implicit_stop(g):
            return g

Regards,
Nick.

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