[Web-SIG] Why is response_headers a list instead of a dict?

[Clark C. Evans]

On Sun, Dec 25, 2005 at 02:13:00PM -0500, Phillip J. Eby wrote:
| In any case, the point is moot; this isn't a compatible change to the 
| spec, so it would have to wait for a WSGI 2.0.

In paragraph #3 of the "start_response()" definition, it states that
type(response_headers) is ListType.  I'm wondering if you'd be willing
to modify this to isinstance(response_headers, list)?

A similar assertion is not made about `environ` parameters, only that 
it is a 'dictionary'.  Could a server or middleware provide a special
environment handler object (as long as isinstance(environ, dict))?

The idea is that these two objects could be customized to provide
low-level RFC support and helper methods; but yet still be 'list of
tuples' and 'dictionary' as required by the WSGI specification.

For example:

  (a) the specialized `environ` could provide attributes which 
      get common HTTP_HEADERs; or raise an error if they do not
      exist -- this would prevent spelling mistakes.

  (b) the specialized `headers` could override the list[selector]
      to take a string argumnet, doing a lookup and replacement; 
      it could also do HTTP Header checking, etc.

Of course, the goal of these objects would be to present the _normal_
dict and list interfaces so that intermediate WSGI applications that
didn't know about the specialization would remain unaffected.

With Python 2.2's __new__ operator, this could be done transparently
at each level, where the intermediate object "adorns" the underlying
native representation.

   my_start_response(status, response_headers):
       response_headers = ResponseHeaders(response_headers)
       response_headers['My-Header'] = 'some-value'
       response_headers.set_content_disposition(filename="bing",inline=True)
       ...

The ResponseHeader class in this case would derive from 'list', and be a
valid WSGI list-of-tuples; for those that know it is a ResponseHeaders
however, they can use the goodness and type-checking provided.  The
implementation of ResponseHeaders() constructor is simple; if the object
is already a ResponseHeaders, it returns self -- otherwise, it
constructs the wrapper as needed.

Kind Regards,

Clark