In mod_python.publisher, the result returned from a published function is returned as the content of the response after it has been converted to a string, using:
In doing this, the second argument of req.write() is defaulting to '1', which indicates that the output should be flushed immediately.
This fact prevents an output filter like CONTENT_LENGTH being used in conjunction with mod_python.publisher.
This output filter will add a content length header to the response, but only if it is able to read the full content the first time the output filter is called. Because the output is flushed at this point, the output filter isn't able to do that, as it gets called twice. The first time it gets called will be with the actual content, the second time happens when the handler returns apache.OK and is effectively a null output call to force the output filter to be closed off.
If instead the output is written as:
the output will not be flushed immediately and instead will be output in one call when apache.OK is returned. There is no loss of efficiency in doing this and instead it will actually eliminate a redundant call into the output filter.
For further details of this issue see discussion in:
This makes one wander if there should be a configurable option for mod_python.psp to tell it not to flush output as well so that CONTENT_LENGTH could be used in that case as well. ???