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

[Guido van Rossum]

> *** frameobject.c       Tue Oct  8 08:16:39 2002
> --- frameobject-222b1.c Tue Oct  8 08:16:12 2002
> ***************
> *** 16,20 ****
>         {"f_builtins",  T_OBJECT,       OFF(f_builtins),RO},
>         {"f_globals",   T_OBJECT,       OFF(f_globals), RO},
> !       {"f_lasti",     T_INT,          OFF(f_lasti)},
>         {"f_lineno",    T_INT,          OFF(f_lineno),  RO},
>         {"f_restricted",T_INT,          OFF(f_restricted),RO},
> --- 16,20 ----
>         {"f_builtins",  T_OBJECT,       OFF(f_builtins),RO},
>         {"f_globals",   T_OBJECT,       OFF(f_globals), RO},
> !       {"f_lasti",     T_INT,          OFF(f_lasti),   RO},
>         {"f_lineno",    T_INT,          OFF(f_lineno),  RO},
>         {"f_restricted",T_INT,          OFF(f_restricted),RO},

A backwards diff!  He posted a backwards diff!  Aargh! :-)

> Is this a reasonable suggestion for 2.3, or is it giving people too
> much rope?  A nasty consequence is that you can write Python code
> that causes Python to seg-fault, but you have to be doing some
> fairly advanced stuff for that to happen.  I would say that the
> power of a Set-next-statement feature in pdb and other debuggers is
> worth the price, but others may disagree...?

I can't approve a Python-level feature that explicitly opens up a
loophole of this kind.  But if you add a C "getset" wrapper that
allows setting this only to safe values (i.e. within the same block
and pointing at the beginning of a bytecode) it might be deemed safe
enough.  It should probably silently update f_lineno as well.  Spare
no cost.  It's up to you to write the patch, submit it to SF, and
prove that it's safe.

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