[Python-Dev] Re: PEP 279 (original) (raw)

Raymond Hettinger python@rcn.com
Sat, 30 Mar 2002 08:55:24 -0500

[Tim]

So I'd like to keep generators simple.

I hear you.

Generator exceptions do less damage (IMO) to simplicity than some of the other extensions, but in practice, in the very few cases I've wanted something like that, this worked fine:

class Whatever: def init(self): self.stop = 0 def stop(self): self.stop = 1 def generator(self): while 1: ... yield something if self.stop: break cleanup

Hmm, the client code needs to maintain TWO instance variables, the generator object and the instance of Whatever. It works but doesn't smell right to me:

w = Whatever() wg = w.generator() wg.next() wg.next() w.stop(); wg.next() # trigger clean-up

Bingo. If you want coroutines, design a coroutine facility. Piling more semantics on to "yield" takes a delightfully simple yet powerful gimmick and hides it under ten kinds of obscurity.

I don't see the obscurity. This is the kind of tool that is invisible and out of the way until the one day you really need it and the tool is right there in your hand (as if by time machine).

Throw wouldn't be used 98% of the time. So, simple things are left simple and hard things are a little less hard. I didn't come up with these ideas out of the ether. It came up in a real world application for producing index prints from jpegs.

That being said, I accept that generator parameter passing and exception passing are doomed.

Here is my alternate proposal that is much simpler and doesn't encourage weirdness. [Developers feel free to stomp on the idea but please don't smack me around for thinking it up].

Specification for Generator Attributes:

1. Allow attributes to be assigned to generator objects.

2.  Provide a means of accessing those attributes from within the
generator by using a reference to __self__.

def mygen(): print self.greeting yield 'hello' print self.greeting

g = mygen() g.greeting = 'Good morning' print g.next() g.greeting = 'Good night'

Advantages: Uses the standard python attribute assignment idiom so there is no learning curve and no surprises. Easily implementable without new keywords, new builtins, or parser magic. Provides a data sharing solution that avoids the yield / next() matching problem, avoids enclosing classes, and avoid global variables. Simple and neat. Disads: Introduces a new system variable, self.

Raymond Hettinger, CPA

P.S. No one needs to say anything mean to make me go away. This is the last generator proposal and then I'm back off to volunteering on Py-Help and co-authoring a Python book update.