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

[Ron Adam]

On 11/22/2014 07:06 PM, Chris Angelico wrote:
> On Sun, Nov 23, 2014 at 11:51 AM, Ron Adam<ron3200 at gmail.com>  wrote:
>> >
>> >
>> >On 11/22/2014 06:20 PM, Chris Angelico wrote:
>>> >>
>>> >>Hmmm, there's no such thing as tuple comprehensions.
>> >
>> >Just didn't think it through quite well enough.  But you are correct, that
>> >would be a generator expression.
>> >
>> >One less case to worry about.:-)
> Ah right, no probs.

>>>>> >>> >And it would be changed to this...
>>>>> >>> >
>>>>> >>> >     def comp_expression():
>>>>> >>> >         for x in itr:          # StopIteration caught here.
>>>>> >>> >             if cond:
>>>>> >>> >                list.append(expr)
>>>>> >>> >
>>>>> >>> >     # StopIteration from cond and expr caught here.
>>>>> >>> >     lst = list(x for x in comp_expression())
>> >
>> >Right, the list.append() should be a yield(expr).

> In that case, your second generator expression is entirely redundant;
> all you want is list(comp_expression()).
Yes, and that is good.  Simplies it even more.

> But the example doesn't say
> *why*  this version should terminate on a StopIteration raised by expr,
> when the statement form would print an exception traceback.

I presume you are asking why do this?  And not why the example does that?

There has been a desires expressed, more than a few times, to make 
comprehensions more like generator expressions in the past.  It looks to me 
that that desire is still true.  I also think there has been quite a bit of 
confusion in these discussions that could be reduced substantially by 
making Comprehensions work a bit more like generator expressions.

As to why the example does that.. a list constructor iterates over a 
generator expression in a way that follows the iterator protocol.  If you 
make comprehsionsons work as if they are a generator expression fed to a 
constructor... then it too should follow the itorator protocol.  Do you agree?


>> >The real question is how much breakage would such a change make?  That will
>> >probably need a patch in order to test it out.
> There's one attached here:
>
> http://bugs.python.org/issue22906

Doesn't that patch effect generators and not comprehensions?  If so, it 
wouldn't do what we are talking about here.

Ron