[mod_python] Passing information through req.internal_redirect().

Gregory (Grisha) Trubetskoy grisha at modpython.org
Sat Apr 23 22:47:31 EDT 2005

How about req.notes - does that work? That may be a more appropriate table 
to use than subprocess_env.


On Sat, 23 Apr 2005, Graham Dumpleton wrote:

> There has been a few times on the list where questions have been asked about
> how to pass information through a "req.internal_redirect()" call. The problem
> is that any data which is stashed in the "req" object isn't available to the
> target of the internal redirect because Apache/mod_python constructs a new
> "req" object for the subsequent handler invocation.
> Turns out there is actually a way of passing at least some information. This
> is done by adding new key/values to the "req.subprocess_env" table. This 
> table
> is held within Apache data structures associated with the request and 
> contents
> of it are propagated through to the target of the internal redirect. The only
> trick is that the key names get modified when the redirect occurs. You also
> can only store string values.
> Take for example a handler which contains:
>  from mod_python import apache
>  import os
>  def handler(req):
>    req.subprocess_env.add("XXX","YYY")
>    req.internal_redirect(os.path.split(req.uri)[0]+'/page')
>    return apache.OK
> and is accessed as:
>  /~grahamd/redirect/redirect
> And a second handler which is triggered as the target of the redirect which
> contains:
>  from mod_python import apache
>  def handler(req):
>    req.content_type = "text/plain"
>    for key in req.subprocess_env.keys():
>      print >> req, key, "=", req.subprocess_env[key]
>    return apache.OK
> That is, URL for this is:
>  /~grahamd/redirect/page
> The result of making a request against the first URL is that redirection to
> the second occurs, with the output being:
>  REDIRECT_REQUEST_URI = /~grahamd/redirect/redirect
>  REDIRECT_SCRIPT_NAME = /~grahamd/redirect/redirect
>  REQUEST_URI = /~grahamd/redirect/redirect
>  SCRIPT_NAME = /~grahamd/redirect/page
> As can be seen, the key/value pair of "XXX" and "YYY" have persisted, 
> although
> the key name is now "REDIRECT_XXX". One can also see that other preexisting
> variables set by Apache for the original request are also propagated as well,
> with similar key name change.
> That one can pass a newly created value would be useful where for example a
> initial handler had created a new session object as it could store the 
> session
> ID in this table such that it is accessible to the second handler. In the 
> past
> people have noticed how in redirects both handlers end up creating separate
> new session IDs if the first handler had to create one, because the new 
> session
> ID is lost and not usable by the second handler.
> The information propagated about the initial request is useful as well for 
> the
> fact that the original URI is present. This could be used by the second 
> handler
> in some way.
> As an example, when using the ErrorDocument directive, Apache uses an 
> internal
> redirect to redirect to the page when it is a local URI. As documented in:
>  http://httpd.apache.org/docs-2.0/custom-error.html
> the req.subprocess_env would contain the "REDIRECT_*" values shown above. In
> this case the internal redirect is being done in Apache itself, but there is
> no reason that you couldn't implement your own version of the ErrorDocument
> functionality in your own custom handler code.
> Anyway, thought I would put together this ramble about this stuff when I 
> found
> that this could be done since it may have been a solution to problems a few
> people had faced before, but that it could be done doesn't seem to have ever
> been mentioned on the mailing list before. At least now anyone searching the
> archives may find this. :-)
> Enjoy.
> Graham
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