[Python-Dev] Let's just keep lambda (original) (raw)

Valentino Volonghi aka Dialtone dialtone at divmod.com
Fri Feb 10 09:19:37 CET 2006

On Fri, 10 Feb 2006 16:49:13 +1300, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:

Valentino Volonghi aka Dialtone wrote:

when someoperationthatresultsinadeferred() -> result: if result == 'Initial Value': when workonresultandreturnadeferred(result) -> innerres: print innerres else: print "No work on result" reactor.stop() Hmmm. This looks remarkably similar to something I got half way through dreaming up a while back, that I was going to call "Simple Continuations" (by analogy with "Simple Generators"). Maybe I should finish working out the details and write it up. On the other hand, it may turn out that it's subsumed by the new enhanced generators plus a trampoline.

This in only partially true. In fact, let's consider again twisted for the example, you can do something like this:

@defgen def foo(): for url in urls: page = yield client.getPage(url) print page

This has 2 disadvantages IMHO. First of all I have to use a function or a method decorated with @defgen to write that. But most important that code, although correct is serializing things that could be parallel. The solution is again simple but not really intuitive:

@defgen def foo(): for d in map(client.getPage, urls): page = yield d print page

Written in this way it will actually work in a parallel way but it is not really an intuitive solution.

Using when instead:

for url in urls: when client.getPage(url) -> page: print page

This wouldn't have any problem and is quite readable. A similar construct is used in the E language and here http://www.skyhunter.com/marcs/ewalnut.html#SEC20 is explained how when works for them and their promise object. You can also have multiple things to wait for:

when (client.getPage(url), cursor.execute(query)) -> (page, results): print page, results

or

l = [list, of, deferreds]

when l -> *results: print results

and we could catch errors in the following way:

when client.getPage(url) -> page: print page except socket.error, e: print "something bad happened"

HTH

-- Valentino Volonghi aka Dialtone Now Running MacOSX 10.4 Blog: http://vvolonghi.blogspot.com New Pet: http://www.stiq.it

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