[Web-SIG] WSGI Open Space @ PyCon.

Graham Dumpleton graham.dumpleton at gmail.com
Mon Mar 30 13:14:23 CEST 2009

2009/3/30 Ionel Maries Cristian <ionel.mc at gmail.com>:
> On Mon, Mar 30, 2009 at 03:13, Graham Dumpleton
> <graham.dumpleton at gmail.com> wrote:
> [...]
>> The problem with this is what happens if a WSGI middleware tries to do
>> something with it. If the separate change is made to allow string like
>> objects to be returned instead of only string objects, then its string
>> like behaviour could be to appear like an empty string. Thus a
>> middleware would see it as an empty string. Of course, the WSGI
>> middleware then may suppress it and not pass it back down the line.
>> As with wsgi.file_wrapper, the problem is that only the WSGI adapter
>> really knows what the type of the iterable instance being returned is,
>> a WSGI middleware can't work it out, except maybe by creating a dummy
>> instance of one and comparing the types. Even then, that may not be
>> guaranteed.
>> This therefore makes it hard for a WSGI middleware to even detect such
>> special control/meta elements and simply pass them through. That also
>> wouldn't work anyway, as a WSGI middleware could be trying to combine
>> together all the strings into one big string to stick a content length
>> on it. In that case it isn't going to even know that a special control
>> element is saying that it should stop trying to do that and instead
>> flush out what it has already accumulated so that underlying server
>> can do other stuff while waiting for file descriptor to be ready.
>> A WSGI middleware doing this sort of thing is going to screw you up
>> even if empty string as might be getting used for this purpose now. In
>> short, I can't see how you can do it this way, as you have no
>> guarantee that a WSGI middleware will not hold on to it. Thus never
>> gets back to underlying WSGI adapter.
> That's wrong - middleware should not eat up response chunks.
> To quote the wsgi spec:
>       To put this requirement another way, a middleware component
>       must yield at least one value each time its underlying application
>       yields a value. If the middleware cannot yield any other value,
>       it must yield an empty string.
> See:
> http://www.python.org/dev/peps/pep-0333/#middleware-handling-of-block-boundaries

In practice I doubt that that is being applied consistently and you
thus can't rely on it.

Even so, any WSGI middleware still needs a way of detecting these
control/meta elements and know it has to pass them through straight
away. They can't just pass back an empty string in place of them as
that defeats the purpose of them.


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