Open Stack

[Reposting my original reply at the very bottom of this, since I sent it from the wrong email address last time and it never hit the mailing list -- for those following along there.]

And Pete, that PEP isn't exactly great and we all definitely translated it differently before. :) Some parts are confusingly worded, it's really long, but at one point in the doc it actually is explicit -- search for "Note: the application". I was pretty surprised when I found that gem and I thought I'd read the whole spec at one point, but had obviously missed that. I'd probably fallen asleep, twice.

-- gholt


On May 21, 2012, at 8:22 PM, Pete Zaitcev wrote:

> On Mon, 21 May 2012 16:06:42 -0500
> Gregory Holt <gholt at rackspace.com> wrote:
> 
>> There are examples of middleware returning generators out there, such as
>> in PEP 333 WSGI, specifically the class example at
>> http://www.python.org/dev/peps/pep-0333/#the-application-framework-side.
> 
> Obviously I didn't read the PEP closely enough, so I assumed the opposite.
> Sorry. Here's my message about it:
> https://lists.launchpad.net/openstack/msg11261.html
> 
>> That is something that swift.common.wsgi.WSGIContext's _app_call works
>> around, ensuring the generator is "activated" before returning.
> 
> I see, I missed that.
> 
> Thanks,
> -- Pete


On May 21, 2012, at 4:06 PM, Gregory Holt wrote:

> Pete, I'm not sure I understand the problem. What do you mean by "middleware should complete by the time __call__ returns"? There are examples of middleware returning generators out there, such as in PEP 333 WSGI, specifically the class example athttp://www.python.org/dev/peps/pep-0333/#the-application-framework-side.
> 
> When a function is called that returns a generator, the generator is returned immediately and no actual code in the function is called until the generator first begins to iterate. This example code might show what I mean:
> 
> def start_response():
>    print 'three'
> 
> def genfunc(sr):
>    sr()
>    yield 'four'
> 
> print 'one'
> gen = genfunc(start_response)
> print 'two'
> for item in gen:
>    print item
> 
> Much code, including older Swift-related middleware, often assumes start_response is called right after calling the genfunc call and before print 'two', but this is not the case. It is delayed until the generator is first iterated over. That is something that swift.common.wsgi.WSGIContext's _app_call works around, ensuring the generator is "activated" before returning.
> 
> Is there some other issue you're having? The newest swift3 now subclasses WSGIContext.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openstack.org/pipermail/openstack/attachments/20120521/7ddc78e2/attachment.html>