Rationale for read-only property of co_code

[Chris Mellon]

On Wed, Apr 2, 2008 at 2:33 PM, João Neves <sevenjp at gmail.com> wrote:
> On Apr 2, 5:41 pm, "Dan Upton" <up... at virginia.edu> wrote:
>  > >  The thing I've been wondering is why _is_ it read-only? In what
>  > >  circumstances having write access to co_code would break the language
>  > >  or do some other nasty stuff?
>  >
>  > >  João Neves
>  >
>
> > I can't speak to Python's implementation in particular, but
>  > self-modifying code in general is unpleasant.  It certainly is
>  > possible to support it in runtime environments, but it's usually not
>  > easy to do so.  That is, of course, somewhat dependent on the
>  > implementation of the runtime environment, and even to some degree the
>  > underlying hardware.  (For instance, the compiled code you want to run
>  > could be in the hardware cache; if you then change the instructions at
>  > those addresses in memory, it's not always straightforward to get the
>  > processor to realize it needs to load the new data into the
>  > instruction cache.)  Plus, I suppose it might be possible to break
>  > strong (even dynamic) typing if you start changing the code around
>  > (although I can't construct an example off the top of my head).
>
>  Indeed, the caching issue is a relevant one I guess, and adding the
>  mechanism to go around that might have a significant impact on
>  performance.
>  I haven't looked in detail into it, but I agree with your opinion
>  concerning strong and dynamic typing. If type check is done while
>  compiling to bytecode, there is no guarantee the modified bytecode
>  will respect the rules, for instance. I don't know though, haven't
>  really checked how it's done at this point. :)
>  I will be fiddling around with the Python VM these days anyway, and
>  since I'm going to muck around the bytecode, I might just try to see
>  the effects of removing the read-only restriction from co_code.
>

There is no need to overwrite co_code. Create a new code object with
your desired bytecode and use that instead.