[Python-Dev] Set-next-statement in Python debuggers

[Guido van Rossum]

> [Guido]
> > Python should not include features (like writable code object
> > attributes) that can cause crashes when used inexpertly.
> 
> If I do implement this, I'll do it in such a way as to make crashes
> impossible (my first suggestion said that crashes would be possible,
> but I realise that was a mistake).  It'll verify that the new value
> of f_lasti is a valid bytecode position, and it'll either fix up the
> stack/blockstack or refuse to move the position in ways that would
> affect them.  (Armin, I guess that won't restrict you because you're
> doing this from C code?)

Great.

> [Armin]
> > For safety I'd consider writing the
> > frame-object-modifying code in a C extension module, carefully
> > documented as "don't use this".
> 
> [Guido]
> > That's a reasonable solution: f_lasti should be read-only from
> > Python code, but you can write an extension that can write it.
> 
> I'd like to see pure Python debuggers (including pdb) have this
> feature.  If it's safe to use, might it still be considered for
> inclusion?  (I can't guarantee I'll have the time to do it, but I'll
> try.)

Sure, *if* it's safe.

> I'm also coming to realise that it's not f_lasti I really want to
> change, but f_lineno.  The line number is the 'unit' that debuggers
> use - a debugger would set f_lineno, and f_lasti would be calculated
> and changed to the appropriate value.  I don't think that changes
> any of the arguments either way, except maybe to distance the idea a
> little from what Armin is doing.

Sure.

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