[Python-Dev] seeing off SET_LINENO

[Tim Peters]

[Michael Hudson]
> 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:
>
> >>> def f():
> ...     if a:
> ...         print 1
> ...
> >>> import dis
> >>> dis.dis(f)
>   2           0 LOAD_GLOBAL              0 (a)
>               3 JUMP_IF_FALSE            9 (to 15)
>               6 POP_TOP
>
>   3           7 LOAD_CONST               1 (1)
>              10 PRINT_ITEM
>              11 PRINT_NEWLINE
>              12 JUMP_FORWARD             1 (to 16)
>         >>   15 POP_TOP
>         >>   16 LOAD_CONST               0 (None)
>              19 RETURN_VALUE
>
> You can see here that the epilogue gets associated with line 3,
> whereas it shouldn't really be associated with any line at all.

It has to be associated with some line >= 3, as c_lnotab isn't capable of
expressing anything other than that.  It *could* associate it with "line 4",
though, if the compiler were changed to pump out another c_lntab entry at
the epilogue.  That would be better than saying the time is charged to line
3, since it isn't on line 3 then.  I'd be happy to trade away total insanity
for partial insanity <wink>.

> For why this is a problem:
>
> $ cat t.py
> a = 0
> def f():
>     if a:
>         print 1
>
> >>> pdb.runcall(t.f)
> > /home/mwh/src/sf/python/dist/src/build/t.py(3)f()
> -> if a:
> (Pdb) s
> > /home/mwh/src/sf/python/dist/src/build/t.py(4)f()
> -> print 1
> (Pdb)
> --Return--
> > /home/mwh/src/sf/python/dist/src/build/t.py(4)f()->None
> -> print 1
> (Pdb)
>
> The debugger stopping on the "print 1" is confusing.

It stops on the "if a:" for me twice today, and I doubt that's any less
confusing.  If it were set to line 4 instead, an unaltered pdb would
presumably show a blank line (whatever) after the function body, and an
altered pdb could be taught that "the last line" c_lnotab claims exists is
really devoted to exit code not associated with any source-file line.