Rationale for read-only property of co_code

Dan Upton upton at virginia.edu
Wed Apr 2 18:41:59 CEST 2008


>  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).

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



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