Rationale for read-only property of co_code

[João Neves]

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.

>
> In short, you need a good reason to support self-modifying code, and
> my guess is nobody could come up with one for Python.
>
> -dan

From what I've seen, Python has some support for lambda expressions,
albeit it may be limited. Self-modifying code would greatly enhance
this feature, I guess. Well, at least from a Lisp newbie
perspective. :)

---
João Neves