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

[Richie Hindle]

Hi,

[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?)

[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.)

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.

-- 
Richie Hindle
richie@entrian.com