[mod_python] Why are some Apache request record fields read only under mod_python

I am trying to re-implement, using mod_python 3.1.3, an Apache 2 
translation handler, originally implemented (by me) using mod_perl. Do 
not ask why unless you want to be bored. And anyway I prefer the Python 
language to Perl.

I appear to be finding members (fields) of the request record which 
have been selected to be read only by the mod_python and thus I cannot 
setup the request record to get the proxy access I need done by 
mod_proxy. For the case in point, with mod_python 3.1.3, the proxyreq 
and handler cannot be set.

What criteria were used in choosing which Apache data structure fields 
were to be read only and why the discrepancy between mod_python and 
mod_perl, both of which are Apache Foundation supported projects, 
presumably with similar objective. And please, do not say the reason is 
to protect developers from their own mistakes.