undefined | Better HN

0 pointschubot13y ago0 comments

Yeah exactly... it escaped me for an embarassingly long time that this is exactly the same scheme used in Lua. In Python you can create a coroutine, then use yield and send(). In Lua you have coroutine.create/yield/resume. They work in exactly the same way.

It was my understanding that implementing "trampolining" as explained by http://dabeaz.com/coroutines/ in Python 2.5 is basically what "yield from" does in Python 3.3. Maybe "yield from" is more efficient. If anyone has any comments on that I'd be interested.

The PIL book has a few interesting examples of things that you can do with coroutines that are not possible with generators (plain yield). Also there is a paper by the Lua authors called "revisiting coroutines" -- although that was published in 2009, they don't acknowledge that Python has coroutines! They talk about Python's "limited" generators.

EDIT: Sorry, "yield from" is for generators. So coroutines are yield/send(). Generators are yield/yield from.

"yield from" is indeed the equivalent of coroutine trampolines, but for generators.

The right way to think about it is that coroutines are a push-based model while generators are a pull-based model. They don't mix, despite using the same keyword! If you want to mix them you would have to introduce a buffer between them or something. This is somewhat explained in Beazley's material.

And coroutines are more powerful than generators, as explained by the "revisiting coroutines" paper (and which should be obvious if you try to transcribe some coroutine patterns into generators.) Even after adding "yield from", which removed one restriction of generators, coroutines are still more powerful.

0 comments

3 comments · 1 top-level

ufo13y ago· 2 in thread

You can also use coroutines for a pull-based model. In the end it just depends on whether you read or write data when you yield and both Lua and Python can do that.

IIRC, the biggest difference between the languages is that Lua coroutines are "stackless" so you can have "non-generator" functions calling "generator" functions. In Python you need to use something like `yield from` and that turns the calling functions into generator functions as well.

chubotOP13y ago

I don't think that's the case. "yield from" is apparently for generators, which allows yielding from "subgenerators" (i.e. a generator can invoke another generator which is part of its stream).

But you can already the equivalent with Python coroutines (and Lua), using the trampolining technique in Beazley's material (and PEP 342).

Python coroutines aren't any less powerful than Lua coroutines. If you still believe they are I would be very interested in an example.

ufo13y ago

Its not about power its about expressiveness. For example, in Python you can't pass a generator function as an argument to `map`, since map doesn't use yields when calling its argument. On the other hand, in Lua the "generator-ness" is a runtime concept instead of a syntactic one so you can pass a generator to `map` and it will still work fine.

I won't bother asking what model is the better one. The Python version is more rigid but its also more explicit...

1 more reply

j / k navigate · click thread line to collapse