[Python-3000] PEP 3132: Extended Iterable Unpacking (original) (raw)

Guido van Rossum guido at python.org
Wed May 2 20:08:36 CEST 2007

[Georg]

>> >>> a, *b, c = range(5) >> >>> a >> 0 >> >>> c >> 4 >> >>> b >> [1, 2, 3]

[Guido]

> Has it been pointed out to you already that this particular example is > hard to implement if the RHS is an iterator whose length is not known > a priori? The implementation would have to be quite hairy -- it would > have to assign everything to the list b until the iterator is > exhausted, and then pop a value from the end of the list and assign it > to c.

[Georg]

Yes, that is correct. My implementation isn't that hairy, though, it's only 13 lines of code more.

OK. The PEP was kind of light on substance here. Glad you've thought about it.

I'll post the patch to SourceForge later today.

Cool.

> it would be much easier if *b was only allowed at the end. (It > would be even worse if b were assigned a tuple instead of a list, as > per your open issues.)

The created tuple is a fresh one, so can't I just copy pointers like from a list and set obsize later?

Sure.

> Also, what should this do? Perhaps the grammar could disallow it? > > *a = range(5)

I'm not so sure about the grammar, I'm currently catching it in the AST generation stage.

Hopefully it's possible to only allow this if there's at least one comma?

In any case the grammar will probably end up accepting *a in lots of places where it isn't really allowed and you'll have to fix all of those. That sounds messy; only allowing *a at the end seems a bit more manageable. But I'll hold off until I can shoot holes in your implementation. ;-)

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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