[Python-Dev] Python advanced debug support (update frame code) (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Mon Jan 13 14:43:35 CET 2014

On 13 January 2014 09:08, Fabio Zadrozny <fabiofz at gmail.com> wrote:

Hi Python-dev.

I'm playing a bit on the concept on live-coding during a debug session and one of the most annoying things is that although I can reload the code for a function (using something close to xreload), it seems it's not possible to change the code for the current frame (i.e.: I need to get out of the function call and then back in to a call to the method from that frame to see the changes). I gave a look on the frameobject and it seems it would be possible to set frame.fcode to another code object -- and set the line number to the start of the new object, which would cover the most common situation, which would be restarting the current frame -- provided the arguments remain the same (which is close to what the java debugger in Eclipse does when it drops the current frame -- on Python, provided I'm not in a try..except block I can do even better setting the the frame.flineno, but without being able to change the frame fcode it loses a lot of its usefulness). So, I'd like to ask for feedback from people with more knowledge on whether it'd be actually feasible to change the frame.fcode and possible implications on doing that.

Huh, I would have sworn there was already an issue on the tracker about that, but it appears not (Eric Snow has one about adding a reference to the running function, but nothing about trying to switch an executing frame: http://bugs.python.org/issue12857).

Anyway, your main problem isn't the reference to the code object from the frame: it's the fact that the main eval loop has a reference to that code object from a C level stack variable, and stores a bunch of other state directly on the C stack.

I don't see anything intrinsically impossible about the idea, it just wouldn't be easy, since you'd have to come up with a way of dealing with that C level state that didn't slow down normal operation.

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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