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Sometimes `print()` and `logging` just aren't enough and you need to get in there and see exactly what's going on. That's where the Python Debugger, commonly known as PDB, comes in.
There are many more commands you can use with PDB.
Reminder: Updating a script while PDB is running will not update the version in your PDB console. PDB doesn't magically reload (it would lose its place if it did).
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One of the major challenges we all go
through as programmers is learning to
0:00
track the state of variables at any given
time.
0:03
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?
6:25
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.
9:01
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.
9:19
If you use pdb, your variable names might
be pdb commands.
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