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One of the major challenges we all go
through as programmers is learning to

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track the state of variables at any given
time.

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For example, if you're working on a list,
pending and

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popping items, it's pretty easy to lose
track of what index a certain value is at.

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Sure, you can print out the list, or use
the logging library to track it,

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but what if there was a better way?

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What if there's a way to get inside your
code?

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What if we could shrink ourselves down
small enough to go inside the computer

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and wait.

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I'm pretty sure that's the plot of a bad
sc-fi movie.

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Probably several of them.

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Okay, but the question's a good one.

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Not shrinking but seeing what's going on.

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What if we could get inside the script
while it's running and

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see the state of everything?

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You know, I wouldn't bring this up if it
wasn't possible.

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This is actually built into python and
it's known as the Python Debugger or pdb.

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Let's go to workspaces and see how it
works.

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All right.
So I made this file called buggy.py and

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it has this list of items that's all
jumbled up.

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We've had a list like this in a previous
code challenge.

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Some of you probably remember that.

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Sometimes you get lists like this from an
API or by capturing user input.

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I mean, it comes from various locations.

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So I want to write some code to modify
this list.

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I want the whole list to be just numbers.

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I don't want anything that is not a
number.

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And we're gonna pretend I don't know any
other ways to do that.

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And that I don't know about the remove
method on lists, okay?

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So yes, this is bad code.

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It's bad code on purpose.

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All right.
So del my_list.

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Let's get rid of number three cuz true is
there.

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And, let's get rid of number four cuz
that's the abcdefg string.

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And let's get rid of six, which is the
false.

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We don't want that false anymore.

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And then let's print out my list.

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I want to clean up this list, and then I
want to print it out.

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Okay.

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Let's give this a try: python buggy.

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Oh, but what's this?

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I've got an index error.

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So I must've counted something wrong.

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So what I could do is I could come back up
here and

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I could print my list and I could do this
after every single step,

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and I could look at what's in there and I
could go oh, that's where I messed up.

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I did something horrible there.

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But that's kind of sloppy, right?

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That kind of takes a long time, it's not
efficient at all.

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So, instead, let's use pdb, let's use the
Python debugger to see what's going on.

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Okay.

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So, the first thing we have to do is we
have to import pdb.

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So, imports go at the top, right?

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So, there we go import pdb.

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And then, wherever in the code that I want
to start my live, my interactive debugger.

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I'm gonna call, set trace.

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So this is like setting a break-point, in
like a JavaScript debugger.

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I wanna jump in and start tracing the file
from right here.

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So I'm gonna do that right before I start
deleting things.

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I'm gonna call pdb.set_trace().

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Okay.

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Everything's set up, yeah?

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It's all set up.

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It's all good.

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So, let's try running the script again.

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Okay so, python buggy.py,

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I didn't get an index there, but, I did
get this weird new prompt,

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I've got this arrow pointing to del my
list 3, then I've got this pdb prompt.

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So, lets deconstruct this a little bit.

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If we look here, there's this six.

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And if we look at our script right there
and we have this

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arrow pointing to del my_list[3] which
just happens to be on line six.

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so this is telling us that this is the
next line we are going to run and

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it is line six.

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Okay, so, my list, del my_list[3]
is line 6.

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That's the next line that's going to run.

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So what we're, what it's doing is it's
letting us know what's coming next, right?

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So, the neat thing about pdb here is that
first of all, pdb has a lot of options.

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We're not going to talk about all the
options.

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There's a pep you can go read.

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There's documentation, tons of stuff.

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But while we're in here,

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it's almost exactly like being in our
standard python shell.

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You know, we go in and we can do like, you
know, help str, and we get stuff, right?

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So, we can do all sorts of things here,

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what's really cool is we can work with all
of our variables.

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So let's see what my list looks like right
now.

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And it looks just like the regular list.

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Right?

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Nothing has happened yet.

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We wanna let it run the next line, so we
can type out next or

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we can just type n for next and press
Enter.

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And there is why you don't wanna use
single-letter variable names.

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What if you set a variable called n?

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And you're trying to check it out.

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There's ways to do it, I'll let you find
that in the doc, but come on,

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that's annoying.

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Okay, so now it tells us that we are on
line seven, and

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that this del my_list[4] is gonna be the
next one.

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But before we do that,

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because we've got these index errors, we
wanna see what's in my_list.

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So we look at my list.

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All right, great, the True is gone which
we wanted to do.

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So we don't want to delete.

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Like this is going to delete number four,
right?

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So zero, one, two, three, four.

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That's the three.

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We don't wanna delete the three.

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We want to delete the string.

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So, let's go and change our script.

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And notice that our pdb did not change.

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Our pb didn't restart or anything like
that.

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It's just gonna keep running.

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So, okay, I wanna, I wanna fix this.

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So let's go in here and let's actually add
in.

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Well, first of all here in pdb, let's
type q to quit.

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So that gets us out of pdb.

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So let's go back up here and

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let's add in some comments so that we can
keep track of what my list is.

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So I'm gonna copy it.

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And I'm gonna come down here.

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Alright.

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So at this point, we deleted true.

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'Kay?

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So down here we're gonna do this three
again, which we

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deleted that one before and its gonna
delete this one now.

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And then, let's go ahead and copy this
down here,

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we wanna get rid of this False.

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So we've got zero, one, two, three, four.

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Alright.
So, we want to del my_list[4].

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And that's gonna get rid of the False.

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Now ideally, if I was actually writing
this,

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I'd go and look for remove or something
like that.

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Some other solution, right?

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You know, maybe I don't think of that.

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Right.

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So let's run our script again.

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So.

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Down here.

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And we're gonna run buggy again.

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'Kay so del my_list?

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Yep, that's good, let's do n.

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del my_list?

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Yep, that should be good.

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Let's do n.

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Yep, that should be good.

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Let's do n.
So the next thing that's gonna run is

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this print(my_list).

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Well let's look at my_list before we print
it.

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And look at that it's just numbers.

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It's exactly what I want.

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'Kay.

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So I wanna let the script just keep
running like it normally would.

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We'll pretend the pdb wasn't even there.

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The way we do that is with using c for
continue.

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Again, you can type out continue.

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We press Enter.

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It prints like it's supposed to and
everything's good.

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'Kay.

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I have one very important thing I
have to remember to do.

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probably the most important thing about
using pdb, and that's I need to get rid of

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it, cause if I leave this in here and I
give the script to somebody else,

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it's gonna hang on them and they're not
gonna know why.

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So let's take that out.

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[BLANK_AUDIO]

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All right.

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So, now, my script is nice and clean.

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I do want to show one more thing,

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though, which is how you will commonly see
pdb used.

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You can do the way that we did it, where
we had the import at the top.

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We bring that back.

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Where you have the import at the top, and
then the set_trace lower down.

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But what you will see most of the time is
this.

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You will see import pdb;
pdb.set_trace(), all in one line.

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Now this is valid.

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It's a little iffy as far as style goes,
but it's perfectly valid.

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This is the only time.

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The only use I will ever tell you where
its okay to use a semicolon in Python.

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Just wanted you to see this and not be
surprised when you see this, otherwise.

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But our script is good,

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we don't need pdb anymore, we wanna get rid
of it and save our file.

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Using the python debugger makes it a lot
simpler to find mistakes in your code, and

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work through exactly what's going on at
any given time.

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Also I'm sure you saw another reason not
to use single letter variable names.

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If you use pdb, your variable names might
be pdb commands.