English subtitles for clip: File:10 - 3 - Worked Exercise Lists (16 35).webm

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Hello, and welcome to yet another worked
exercise.

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I'm the author of the book Python for
Informatics: Exploring Information,

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as well as your host for this
particular worked exercise.

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As always, the book, this audio and video,

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and these slides are copyright Creative
Commons Attribution, and

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I hope you find exciting and interesting
ways to

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reuse them and re-purpose them and add to
them.

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So pythonlearn.com is my support website
for the Python for Informatics book.

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And if you, hopefully by now you've got
all this

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worked out, but it does teach you how to
get started.

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So that, how all the things that you need
to know,

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to edit files and use the command line to run
Python programs you've got figured out.

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Okay?

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So our program for today is from the list
chapter.

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And our program is actually, we're
going to debug a program

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rather than just writing a program from
scratch.

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And so what our, our task is, in this
program is we're looking

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for words that start with from,
looking for lines that start with from.

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And on the lines that start with from,
we're going to pull out the

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day of the week that this particular

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email message was sent. Saturday, Friday,
whatever.

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And so the structure of the program
is pretty standard.

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We're going to open a file at the top.

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We're going to loop through the file.

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Strip the newlines at the end, right?

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There's a little newline at the end of
each line.

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And then we're going to use split to split
it into words,

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which means that'll make this the
zero word, the one word,

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and the two word.

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And then we're going to check the zero
word to see if it's from,

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and if the zero word's not from we're
uninterested in that line so we're going

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to skip back up. And if it is good, if we
wfind a line

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we're interested in, we're going to skip
the majority of the lines in this file.

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Less than one in 100 of these lines
actually

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have from in the, as the first word.

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And then the second word is what we're
interested in

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so we're going to print the, the day of
the week.

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So this program as we type it in is going
to have a

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traceback and we go like, you know, as soon
as you see traceback

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you immediately are drawn to the word
traceback and maybe you've gotten to

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the point where you start to read the why,
list index out of range.

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And then, you know, it says line five, so
that's kind of helpful.

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But the first thing to not get distracted
about is, is this.

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Our program actually ran a little bit.

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So when you look at a traceback, look
right above it and make sure, and

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maybe you won't see any output, but maybe

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your program will have partially run,
and so

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don't immediately assume the program is
totally broken. So this

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is actually successful output of one of
the many from lines.

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And it, it dies later.

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And so, as we debug it, we'll sort of come
up with techniques to sort of figure out

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how much your program has done before it
died,

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because that might be an important,
important question to answer.

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How far did your program get, because if
it dies before the first line, or on

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the first line, that's something different
than if

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it goes like 300 lines and then dies.

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If it goes a long ways, 300 lines, and
then dies,

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maybe there's something subtle or weird
about the line it's dying on,

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rather than just your program is

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just like totally broken.

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So you got, look for something where your
program actually partially worked,

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and helps you kind of narrow down
your suspicions as

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to what might be wrong with this.

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So let's get on with the, let's get on
with the programming.

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And so I will simply steal this.

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I'll cut and paste it.

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And that will be that.

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So I'm going to cut and paste this into my
TextWrangler.

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Now it's always dangerous cutting and
pasting.

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But I'm cutting and pasting from a slide
which seems

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to work a little better than cutting and
pasting from PDFs.

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And you can even get away with cutting and

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pasting with PDFs as long as you don't
freak out

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when the first thing you see is a bunch

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of crazy syntax errors because of
characters being coded improperly.

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So now I'm in this file, and I'm going to
save this.

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Save As, and I'll put this on my Desktop

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in the lists folder.

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And we'll call this day of the week .py in the
lists folder.

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Okay, and of course it's, syntax

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colored it, which makes my TextWrangler

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syntax colored it, and so I've got sitting

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in this lists folder, I've got the
day of the week .py.

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Now one of the things that we've got to do
is we've actually got to get our data file.

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See this mbox-short.txt?

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So the mbox-short.txt is sitting here on
pythonlearn.com.

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I'll go to the book page and you'll see
this, all the stuff and code samples.

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Including maybe a worked version of this
example, you just never know.

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[NOISE] Ii doesn't look like
this one's here but

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the file I'm looking for is mbox dot
short dot t-x-t.

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So there it is.

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It's a mailing thing and here's one of
those lines

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we're looking at and we're going to
there's many of them.

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This happens to be the first line of the
file.

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So I'm going to make sure that I save this
into the folder.

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Go back to my Desktop and put it in lists
and then save it.

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And so.

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At that point I should be able to go back
and see ah-hah, I have mbox-short.txt.

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Now the interesting thing is if you read
this file, [SOUND] if you open this file,

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TextWrangler's perfectly happy.

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So we see from.

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So if I was to do sort of a search, and I
want to search for the string

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capital from space. Okay?

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And then I'm going to search next.

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Oh, theres lots of them.

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I'm going to make them case sensitive,
okay?

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So it'll only be the froms that are case
sensitive.

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There we go, and so in a sense our program is
doing this little from where it's looking.

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It's throwing away all the lines except
those that start with

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from and a space, because of the way the
split works.

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And then it's going to pull these, these
days of the week, these Fridays.

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So I'm kind of doing, if I was doing it by
hand it

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would be like find the line that starts
with from blank, and then

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grab this little text, and that's what I'm
interested in.

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Maybe I'm curious as to whether or not
these

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folks work on the weekends or on the
weekdays.

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And that's the purpose of this.

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Okay?

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So I'll close this.

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Just, it's, that's what, that's the, Close
Document.

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There we go.

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So here's my little program.

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And so, let's, I can get rid of this now.

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So now I'm going to go into my Desktop.

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I'm in my home directory.

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cd Desktop, cd lists. If I do ls, there
I am.

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I've got this day, day of the week Python
program, and mbox-short.txt sitting here.

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So I'm going to run it [SOUND]
and there it works.

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I'll get, I mean, well, works for some
value of work.

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So we get the same traceback the lecture
slide suggested

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we would, and thankfully that probably
means the lecture slide's right.

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So, so here we go.

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We got, we got this one thing where the
day of the

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week is kind of coming out and then it
dies on line five.

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And it even gives us the line it's dying on.

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So, it's complaining about list index out
of range, and you

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might be able to stare at this and maybe
you're smart.

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You're, you have enough skill already that

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you're seeing these kinds of errors and
you

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just read it right away, know what the
problem is, but that's not so much fun.

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So here I am in line five, right?

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I'm going to go right to line five.

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And, and so the first thing I want to do
on line five,

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is I often add a print statement right
before the line that's dying.

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And I'll just print out something [SOUND].

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Something random.

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Just, it, you know, sometimes I print the
letter a out, right?

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And so now I'll save this.

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And now I'm going to run it again,

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and this tells me, just by putting a print
statement in, what's going on here.

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It's like, okay wait a sec.

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This is the good line.

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This is the line, I mean, this is the line
I'm interested in.

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As a matter of fact, I'm going to make
another change

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just to help me sort of visually see
what's going on.

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[SOUND] I'm going to put like a bunch of
equal signs at

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the beginning of this line and now I'm
going to run it again.

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So if I look, it's like, oh, dude, that's
the good line.

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That's the line I'm liking.

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And here's my debugging, Something random,
Something random, Something random.

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So a lot of stuff is going on here.

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And then finally it dies, right?

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So finally, it dies here.

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Now, it's line six because I added a line.

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So the, so perhaps instead of printing

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out Something random, I'll print something
useful.

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So, the first thing to do is to look at
this statement, the one that's dying.

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And say, what is the most suspicious thing
in here, okay?

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And, and so, if something's going wrong
with this

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words sub zero, it's saying index out of
range, right?

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Something going wrong with words sub zero.

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So what's the deal?

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So I'm not going to print out words sub
zero, I'm going to print out words.

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I'm going to say, like, what is in this
words list at this point?

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So now let me save that.

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So instead of printing something random,
we'll see a bunch of words go by.

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And so here's that first line broken

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into words, where from Stephen Marquard,
Saturday.

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And here's our good line.

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And then here's one that doesn't have from
as the first word, so we skip it.

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Here's one that doesn't.

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So let's continue onto where it's, you
know,

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we see lots of lines, everything's
cruising along.

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We actually see one that says from colon,
but that's not what we're looking for.

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So that one gets skipped too.

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So make sure you skip that.

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Now, here we are.

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Here's the words.

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Here's that list of words that we've
split.

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Now, this was the line before, here's the
next line.

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It's like, okay, what's going on here?

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That looks like an empty list.

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Okay, so I'm a little more curious, I'm
going to,

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I'm going to print, I'm going to print
line out too.

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[SOUND] And I'm going to put, just so I
know what's

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going on, I'll stick some, a couple pluses
in front of that.

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00:10:12,120 --> 00:10:14,590
So the line will have pluses on it, and
it'll print the word.

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So let's run this again.

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So I'm adding junk to this, right?

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So now when I look at my things, the
pluses mean that's the line I'm seeing.

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Right?

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I just put those pluses in for my own

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visible, so I can visually see it more
naturally.

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That's the line and you can see how nicely
it breaks

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it into words, so it breaks this line at
the space.

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Oops, sorry.

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That was not so good.

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It, it breaks this line, spam probability
line, at this space and gives me two words.

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Right?

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So here we are.

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Two, here we go.

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Here is this space, and then it breaks
it into one word, and another word.

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And so that's working.

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But if I look here, that's the line that
I'm on.

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Oh, well that's a blank line.

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Hang on a sec, hang on a sec. So let's

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just real quick take a look at that file
again.

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Let's look at that file, and so here we
go, here we go

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So that's cool, that's cool, that's cool,
let's look at the line.

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00:11:13,640 --> 00:11:16,220
And here we go, so if you look.

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The last thing that we read successfully
was this line right here.

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This line is a blank line, so our program
is choking on blank lines.

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It's not failing on lines that work right,
and it's

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not failing on lines that don't have from
in them.

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It can handle the front line fine, it can
handle the, any other line fine.

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00:11:36,350 --> 00:11:41,640
But it freaks out on the simplest of
things, blank lines.

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So the question is, how do we fix this?

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[SOUND] Right?

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How do we deal with this fact that
this bit

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of code, right here, is dying whenever
there's a blank line, okay?

231
00:11:57,530 --> 00:12:00,135
So, the way we do this is what's called
the guardian pattern.

232
00:12:00,135 --> 00:12:02,180
[SOUND] I'm going to get rid of this.

233
00:12:03,910 --> 00:12:07,210
So one of the guardian patterns is it
looks like this.

234
00:12:07,210 --> 00:12:08,770
So, we don't want to get to this.

235
00:12:08,770 --> 00:12:10,350
So we want to put something in front of it
to

236
00:12:10,350 --> 00:12:13,180
make sure we never hit it in a dangerous
situation.

237
00:12:13,180 --> 00:12:17,370
So it's if hmm, hmm, hmm, something,
continue.

238
00:12:17,370 --> 00:12:24,730
And this is if, we'll call this thing the
safety check. [SOUND]

239
00:12:24,730 --> 00:12:25,370
Check.

240
00:12:27,240 --> 00:12:29,330
If some safety check, continue.

241
00:12:29,330 --> 00:12:35,210
So that means that, you know, you know, if
the safety check matches,

242
00:12:35,210 --> 00:12:38,370
we're going to not fall through and not do
this dangerous line.

243
00:12:38,370 --> 00:12:40,830
So the question is what would we put in as
the

244
00:12:40,830 --> 00:12:45,390
safety check here to protect. This line is
guarding this line.

245
00:12:45,390 --> 00:12:47,470
That's what the guardian pattern means, is do

246
00:12:47,470 --> 00:12:50,130
something before the scary thing that
hurts you.

247
00:12:51,520 --> 00:12:52,111
Okay?

248
00:12:52,111 --> 00:12:59,060
So, one thing we can do is we can say,
okay, well, what's words?

249
00:12:59,060 --> 00:13:00,700
Well words is an empty list.

250
00:13:00,700 --> 00:13:01,620
You know what?

251
00:13:01,620 --> 00:13:05,820
If, if I got an empty list of words, I
have no interest in this.

252
00:13:05,820 --> 00:13:11,340
So if I say words is equal to an empty
list, continue.

253
00:13:11,340 --> 00:13:12,526
So what this basically says is.

254
00:13:12,526 --> 00:13:13,097
You know.

255
00:13:13,097 --> 00:13:15,148
You know.

256
00:13:15,148 --> 00:13:18,630
Read the line, strip it, split it into
words.

257
00:13:18,630 --> 00:13:19,630
We'll print it.

258
00:13:19,630 --> 00:13:26,230
If it's an empty list, continue and go up
to the next line.

259
00:13:26,230 --> 00:13:29,770
And then, if it is a non-empty list, it
continues here.

260
00:13:29,770 --> 00:13:33,010
And checks to see if the first word is
From.

261
00:13:33,010 --> 00:13:36,110
And if it is not, it continues, and then if
it works, it works.

262
00:13:36,110 --> 00:13:40,850
So this here, this line here, is
the guardian line, it's

263
00:13:40,850 --> 00:13:44,240
guarding the, this other line and you
gotta do it in order.

264
00:13:44,240 --> 00:13:45,220
If you do the guard, you have to

265
00:13:45,220 --> 00:13:48,650
have the guardian happen before the line
in question.

266
00:13:48,650 --> 00:13:49,520
So now let's run it.

267
00:13:52,360 --> 00:13:53,850
Oops.

268
00:13:53,850 --> 00:13:56,280
Let's run it, and now it works great.

269
00:13:56,280 --> 00:14:02,860
It's, we got a little too much crap, so we
better get rid of this print statement.

270
00:14:04,150 --> 00:14:05,570
It didn't give me a traceback, right?

271
00:14:05,570 --> 00:14:08,690
So now I'm going to get rid of that print
statement and look at that.

272
00:14:08,690 --> 00:14:11,900
I'm getting the line Saturday, Friday,
Friday, Friday, Friday, Thursday.

273
00:14:11,900 --> 00:14:14,140
So it doesn't look like these people work
weekends.

274
00:14:14,140 --> 00:14:18,320
But it does look like they like finishing
up stuff toward the end of the week.

275
00:14:18,320 --> 00:14:19,620
So, there we go.

276
00:14:19,620 --> 00:14:24,000
I mean, it's a little bit of, it's not
much data to conclude anything solidly.

277
00:14:24,000 --> 00:14:28,140
Maybe if we look at the bigger mailbox
file, we can find something else.

278
00:14:29,300 --> 00:14:30,590
Okay, so now it works.

279
00:14:30,590 --> 00:14:32,930
Let me show you a couple of other things

280
00:14:32,930 --> 00:14:34,890
that you could have used as a guardian
pattern.

281
00:14:34,890 --> 00:14:37,910
So we could say  if the words we got back is
an empty list.

282
00:14:37,910 --> 00:14:43,640
I could say if the number of elements in
words is less than one, continue.

283
00:14:43,640 --> 00:14:47,680
That basically says, look, if the words
that I got, if I got fewer than

284
00:14:47,680 --> 00:14:49,230
one word, then I'm not going to look

285
00:14:49,230 --> 00:14:51,680
for the first word, because that's quite
dangerous.

286
00:14:51,680 --> 00:14:54,810
So if I run that, it again works.

287
00:14:54,810 --> 00:15:03,280
Another more direct, even potentially more
direct, would be if let's move this up.

288
00:15:06,770 --> 00:15:13,630
I could say if line is equal to empty
string, continue.

289
00:15:14,690 --> 00:15:17,590
So now what it says is, you know, if I'm
getting a blank line,

290
00:15:17,590 --> 00:15:19,220
I'm not even going to bother splitting it

291
00:15:19,220 --> 00:15:21,240
because I know exactly what's going to
happen.

292
00:15:21,240 --> 00:15:22,455
So let's see if this works.

293
00:15:29,455 --> 00:15:31,560
[SOUND]
So this one works as well.

294
00:15:31,560 --> 00:15:37,310
So now I've got, you know, a couple of
codes, a couple of code paths through here.

295
00:15:39,030 --> 00:15:43,650
Right? If, if the blank line, I do it after
the strip to make sure it is really empty.

296
00:15:43,650 --> 00:15:47,060
I skip blank lines the way, I come all
the way down

297
00:15:47,060 --> 00:15:50,360
and split and then skip lines that don't
start with from this way.

298
00:15:50,360 --> 00:15:53,280
And the only way I make it all the way
down to here is if it is

299
00:15:53,280 --> 00:15:58,620
a non-blank line and if the first word is
a From, and then I do my thing.

300
00:15:58,620 --> 00:16:02,370
So that basically is the notion of

301
00:16:05,230 --> 00:16:08,330
the guardian pattern, and the pattern is
simply,

302
00:16:11,340 --> 00:16:15,980
if there is some code that might have a
problem depending on perhaps user input,

303
00:16:15,980 --> 00:16:18,200
put something in before it

304
00:16:18,200 --> 00:16:20,755
that makes sure that you never get to the
dangerous code.

305
00:16:20,755 --> 00:16:22,980
And don't use a try/except for this.

306
00:16:22,980 --> 00:16:27,356
That just would be tacky, tacky, tacky,
tacky.

307
00:16:27,356 --> 00:16:32,600
So that's the end of this this
presentation.

308
00:16:32,600 --> 00:16:34,860
So thanks a lot, and see you on the net.