[Python-Dev] seeing off SET_LINENO

[Guido van Rossum]

> I've found another annoying problem.  I'm not really expecting someone
> here to sovle it for me, but writing it down might help me think
> clearly.
> 
> This is about the function epilogues that always get generated.  I.e:
[...]
> The debugger stopping on the "print 1" is confusing.
> 
> There's an "obvious" solution to this: check it we're less than 4
> bytes from the end of the code string and don't do anything if we are.

Um, I think that's less than reliable.  I believe we just discussed
this when Oren's patch for yield in try/finally did a similar thing
(and weren't you the one who mentioned that your bytecodehacks can
cause this assumption to fail? :-).

I'm not actually sure that this needs fixing.  Surely the --Return--
should be a sufficient hint.  I note that without your patch it also
stops at a confusing place, albeit a different one (on the "if a:"
line).

> This would be easy, except that for some bonkers reason, we support
> arbitrary buffer objects for code strings!  (see _PyCode_GETCODEPTR in
> Include/compile.h -- though at least you can't create a code object
> with an array code string from python, the getreadbuffer failing will
> cause the interpreter to unceremoniously crash and burn).

That went a little too fast.  Can you explain that parenthetical
remark more clearly?

> I guess I can store the length somewhere -- _PyCode_GETCODEPTR returns
> this, more by accident than design I suspect -- or call
> bf_getsegcount(frame->f_code->co_code, &length) or something.
> 
> Does anyone actually *use* this feature?  I see Guido checked it in
> and the patch was written by Greg Stein.  Anyone remember motivations
> from the time?

Yes, Greg insisted that he might want to store Python bytecode in
Flash ROM, and that this way the bytecode would not have to be copied
to RAM.  But I don't think this ever happened (well, maybe the
now-dead Pippy port to PalmOS used it???).

I'd be happy to kill it as a YAGNI.

But that still doesn't mean I approve checking for "4 bytes from the end".

--Guido van Rossum (home page: http://www.python.org/~guido/)