[Python-ideas] "Iteration stopping" syntax [Was: Is this PEP-able? for X in ListY while conditionZ:]

[Andrew Barnert]

On Jun 30, 2013, at 15:08, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:

> Ron Adam wrote:
>> It's the same as in-lineing the generator parts into the iterator that is driving it.
>> We don't need to do that because we already have an optimised version of that.  It just needs to catch the StopIteration to be the same.
>> I think that it's not uncommon for people to think this is how list comps work.  And I think it is surprising for them that the StopIteration isn't caught.
> 
> I tend to feel that the fact that raising StopIteration in a
> generator has the same effect as returning from the generator
> is a quirk of the implementation that shouldn't be relied on.
> I'm not sure we should be giving it official status by going
> out of our way to make listcomps behave the same.

What other effect could it possibly have?

The genexp doesn't do anything special with a StopIteration--it passes it through like any other exception. And this is the same as for an explicit generator function. The calling code--whether it's calling next(), using a for loop, or iterating in C--sees StopIteration for a generator return by definition, and sees StopIteration if explicitly raised because that's how exceptions and generators work.

Unless you add an extra new rule that says raising StopIteration inside a generator is illegal and will raise a TypeError or something, there's no other way a valid implementation of Python could possibly work. 

I think a lot of people are looking at this wrong. In list(genexp), it's not the genexp part that's doing anything here; it's the list function. There's no way that it could distinguish between an explicit StopIteration and an implicit one, so it treats them the same way. If a comprehension is supposed to be the same as list(genexp) it should act like list--or like any other iteration mechanism you can write in Python or even in C--rather than acting magically as it currently does.