[Python-Dev] seeing off SET_LINENO

[Michael Hudson]

barry@python.org (Barry A. Warsaw) writes:

> >>>>> "MH" == Michael Hudson <mwh@python.net> writes:
> 
>     MH> 3) The patch changes behaviour -- for the better!  You're now
>     MH> rather less likely to get the trace function called several
>     MH> times per line.
> 
> Does this change affect debugging?

Hmm, I hadn't actually dared to run pdb with my patch... have now, and
it seems OK.

There is a difference:

The bytecode for, say,

def f():
    print 1

begins with two SET_LINENO's.  One is for the line containing "def
f():", one is for "print 1".  My patch means the debugger doesn't stop
on the "def f():" line -- unsurprisingly, given that no execution ever
takes place on that line.

It would be possible to force a call to the trace function on entry to
the function.  In fact, there's a commented out block for this in my
patch.  Another approach would presuambly be for pdb to stop on 'call'
trace events as well as 'line' ones.  I don't really understand, or
use all that often, pdb.

Also, you currently stop twice on the first line of a for loop, but
only once with my patch.  There are probably other situations of
excessive SET_LINENO emission.  I know Skip (think it was him) killed
a couple last week.  Bug compatibility is possible here too, but I
don't see the advantage.

> Have you tested how this change might interact with e.g. hotshot?

test_hotshot was very important to me as evidence I was making
progress!

It currently fails due to the not-calling-trace-on-def-line issue, but
as I said, I think this is a *good* thing...

Cheers,
M.

-- 
  The ability to quote is a serviceable substitute for wit.
                                                -- W. Somerset Maugham