Time Travel Debugging (in Python)

This video introduces the time traveling debugging for Python I've been working up to today. It show what it's like to code using time traveling debugging, how my implementation works, as well as a proof of concept debugging UI called the zoom debugger. The time traveling debugger project can be found at: https://github.com/airportyh/time-traveling-debugger.

Transcript

The following transcript was automatically generated by an algorithm.

00:00 : let me show you how time travel
00:01 : debugging works
00:03 : let's say you get a weird error like
00:04 : this one
00:07 : what you can do now is go back to the
00:09 : point in time when this error happened
00:11 : and look at the state of the program
00:14 : x not in list what is in the list
00:18 : oh right we have this bracket
00:22 : so zero is not going to be in the list
00:26 : oh what i probably meant to do is remove
00:28 : the item at index zero
00:31 : not remove the item zero it gets better
00:35 : you can also rewind and replay the
00:37 : events leading up to the error
00:41 : assertion error okay let's step
00:44 : backwards into the function call the
00:47 : return value is zero
00:50 : uh we're expecting it to be negative 42.
00:53 : stepping backwards so char
00:57 : is the negative sign which is
01:00 : clearly not a digit oh right i never
01:03 : implemented logic that handles negative
01:06 : numbers
01:07 : oops so let's set up a sine variable
01:11 : and we're gonna set it to either
01:13 : negative one
01:14 : or one depending if we get a negative
01:17 : sign
01:18 : let's go still know
01:23 : what's going on stepping backwards
01:26 : uh looks like we have set up the sign
01:28 : correctly
01:29 : oh okay you just forgot to multiply the
01:32 : sign into the result
01:34 : duh what you're seeing
01:37 : is a time traveling debugger prototype
01:39 : for python programs
01:41 : how does it work well first to run your
01:44 : program
01:44 : instead of using the standard python 3
01:47 : executable
01:48 : you use pi rewind a customized version
01:51 : of python 3.
01:52 : pi rewind creates a log file which
01:55 : leaves a trail of everything that
01:56 : happened during the running of the
01:58 : program
01:58 : such as function calls variable
02:00 : assignments and updates to data
02:02 : structures such as
02:03 : lists and dictionaries then the recreate
02:07 : program reads back the log file
02:08 : line by line and recreates every
02:11 : historical state of the program
02:13 : and saves them into a sql lite database
02:16 : we call this database file the history
02:18 : file
02:19 : and it can be used to drive a debugger
02:21 : front end such as this one
02:24 : on the surface this looks like a normal
02:26 : step debugger but
02:27 : it's not executing the code line by line
02:30 : but rather it's just navigating the data
02:32 : in the history file
02:33 : because of that it has the ability to
02:35 : step backwards in time
02:40 : the fact that you have access to every
02:42 : point in time in history
02:44 : gives rise to some interesting
02:45 : possibilities one possibility
02:48 : is you can use database queries to
02:50 : answer questions about what your program
02:52 : did
02:54 : another possibility is you can
02:55 : re-imagine what a debugger's user
02:57 : interface can be
02:59 : this is a debugger front-end called the
03:01 : zoom debugger
03:03 : it was inspired by the mandelbrot set
03:05 : and google earth
03:06 : within it you can look at a program's
03:08 : execution from different levels of
03:10 : elevation
03:12 : the top level is the module the entry
03:14 : point of every python program
03:16 : if
03:22 : we have a hero and we have two enemies
03:26 : the goblin and the wizard there's a
03:29 : battle engine
03:30 : a shopping engine and then it's looping
03:34 : through the
03:34 : enemies the goblin is the first enemy
03:38 : and looks like a battle happened here
03:41 : and the aftermath hero has health 8
03:44 : after the battle
03:46 : goblin has held negative four
03:50 : down from six over here
03:54 : and we did some shopping after the
03:57 : battle
03:58 : and then we do the second battle with
04:00 : the wizard
04:02 : uh we beat the wizard too
04:05 : and then we did some more shopping at
04:07 : the end
04:09 : and we won all right let's zoom into the
04:11 : first battle
04:14 : okay so hiro attacks the enemy
04:18 : uh the goblin and the goblins health was
04:21 : down to
04:22 : one after just one single attack
04:26 : how is that determined let's go in
04:30 : so to attack an enemy we have to first
04:33 : ask the enemy to receive the damage
04:37 : interesting uh here we're asking them to
04:40 : receive
04:41 : self that power self is the hero
04:44 : and hero's power is five okay so let's
04:47 : go in again
04:49 : so now we're inside the receive damage
04:52 : method of the goblin
04:53 : and we're just going to subtract the
04:56 : points
04:57 : from our health so since we started with
05:00 : a held of
05:01 : six now we're down to one
05:04 : that makes sense imagine if you could
05:07 : see a visualization like this while
05:09 : working with
05:10 : or reviewing a new piece of code you
05:12 : haven't seen before
05:13 : the prototype you saw is pi rewind and
05:16 : is a part of the time traveling debugger
05:18 : project which you can find here
05:20 : the concept of time travel debugging is
05:23 : an active area
05:24 : of research there are active projects
05:26 : commercial products as well as research
05:29 : papers in this area
05:31 : links to these can be found on the
05:32 : project page as well
05:34 : if you like to experience what it's like
05:36 : to code with time travel debugging
05:38 : try using pi rewind to solve a code
05:41 : challenge on lead code or a similar site