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Handle Exceptions
9:05Let's catch a couple of exceptions and raise one of our own.
I'm going to clean up these comments,
they're getting a little busy,
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these instructional ones.
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So we'll get rid of this,
so I'm using again,
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Command + Shift + D to
delete some lines here.
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Let's bring this up.
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Don't need that anymore,
don't need that or that or those.
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Let's get rid of that,
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I'm going to leave that to-do because
we definitely need to do that later.
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Get rid of this.
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Otherwise, and thanks.
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Sold tickets.
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This line, there we go.
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And it's actually 20 lines of code,
awesome.
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That feels better.
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I'm gonna save it, and now,
when we get this cleaned up,
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should we walk this real
quick with those missing?
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Remember, that we are on the,
as a user I should have errors reported in
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a user-friendly manner,
which is kind of always the case.
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But we really wanted to make sure that
we handled this cuz they saw one of
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the tracebacks that I left earlier.
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So all right, here we go.
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First off.
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So here, we have a constant and it's in
all upper case, it won't change while
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the program's running and if the group
wants to change this in the future,
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it's a very easy place to change it,
constants.
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So these are how many tickets
are currently available, now note,
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it's not all uppercase.
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So we can assume that
this is going to change.
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And then we have, while there is one or
more tickets remaining.
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Okay, we're going to keep on
living through all this code.
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So we're going to show how many are left.
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Then we're going to grab their info,
what's your name, how many tickets,
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we're going to coerce that.
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Because input always returns a string, but
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we need to use num_tickets for
a math purpose.
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So we definitely need an integer.
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Now, it's an integer because you
can't buy fractions of a ticket.
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I'd like to buy one and
a half tickets please, that doesn't work.
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So if you remember, this seems
a little bit dangerous, doesn't it?
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What if the user gave us
something that we couldn't coerce?
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Let's keep this in mind for
a possible problem.
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Okay, and then we calculate the price.
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And then we show them the total.
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And we ask them, yes or no,
do you want to proceed?
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And because we aren't sure if they
are going to use an uppercase Y or
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lowercase in responding, we lowercase it.
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And we check.
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We left a nice to-do message here that
the SOLD is not really taking any credit
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cards, and then we subtract the number
of tickets from the tickets remaining.
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What would happen if the user
entered 1,000 tickets?
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Whoa, that'd be negative
900 tickets remaining.
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The while loop up here,
this would stop, right?
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Cuz it'd be negative 900, that would stop.
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It's not greater than or equal to one, but
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still, the 1,000 ticket
sale would go through.
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We've got to catch that, right?
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So and then finally,
anything other than y,
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we're going to say Thank you anyways, and
we're going to be friendly about that.
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And then we're going to let people
know that those tickets are sold out,
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because the loop's over.
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So once that loop's over, they know.
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Did that feel pretty good being
able to read most of that?
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That's pretty cool, right?
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Pretty amazing.
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All right, now,
let's take care of these errors.
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Let's do one at a time.
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So the first problem that we have is here,
one around coercion, right?
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So remember,
the user can add anything here.
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So let's run it to see how
bad the errors actually are.
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I want to clear this.
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Let's run python masterticket,
and our name is Gallahad.
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How many tickets would you like?
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Blue.
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Blah, what a gross error.
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And our program ended.
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We can't have that happening.
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We won't be able to sell anything
more if we drop out like that.
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So it throws a ValueError.
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I'm gonna go ahead, I'm gonna copy that.
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And right above here, right above
int( num_tickets), let's say expect,
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I'm gonna paste a ValueError to happen.
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And handle it appropriately.
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Let me give you a hint,
remember to test it out.
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Pause me, and give it a go.
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Are you ready?
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All right, ready?
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This is how I went about fixing it.
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So I definitely needed a trial block.
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So I did the block here and
we wanna be careful about the num_tickets.
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And I want to accept,
we know that that throws a ValueError.
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And what we wanna say is print,
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no, we ran into an issue,
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please try again.
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Now, that feels good, doesn't it?
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But I'm gonna go ahead and run this.
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My name is Gallahad and I would like.
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How many tickets would you like blue.
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Total due is blue blue blue blue blue.
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What?
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How is blue blue blue blue blue.
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Yes.
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Remember, this is where the error happened
and the assignment never happened.
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So num_tickets here is the string blue.
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And because you can multiply strings,
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right, repeat the same
thing a number of times.
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This repeated however marks
the ticket price was which is ten, so
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those are ten blues right there.
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So in order to fix that,
I need to put this code in an else.
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Cuz we only want this code to run
if we know how many there are.
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So let's do this, and I'm going to
indent it with command right bracket.
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Then let's go ahead, and if you want
to ever stop out of a program while
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it's waiting at a prompt, you can do a
control C and it will send this interrupt.
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So let's run that one more time,
make sure it's working.
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My name is Gallahad, blue.
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It says, no, we ran into an issue,
and then it just keeps on looping.
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There's 100 tickets remaining, awesome.
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Okay, now we have another error,
too, don't we?
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We don't want to allow for
that 1,000 ticket sale, right?
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We need to block that.
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So why don't we do that
in the try portion here?
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So if we come up here into our try,
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we wanna make sure that we have a valid
number of tickets to request, right?
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So we will raise a ValueError
if the request is for
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more tickets than are available.
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And then I wanna make sure that that's
a user friendly error that we raise.
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So what I want to do next is I
wanna include in the output here,
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we want to include the error
text in the output.
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All right.
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Remember, you can lean on the community.
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And also, you can rewatch the previous
video where we did something similar,
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check the Teacher's Nets.
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You got this.
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Are you ready?
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Pause me.
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Ready?
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So here's what I did.
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So I used an if statement up here.
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So I said, if the number of tickets,
that's what they asked for is greater
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than the tickets_remaining, you can't
do that, you can't buy those tickets.
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So, what we did was we
raised a ValueError.
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And I'm gonna go ahead and
be very specific, there are only,
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use a placeholder here, tickets remaining.
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Nice try.
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So then we'll do format and
we'll say tickets remaining,
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and then we'll do a closing.
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Okay, and
then if I want to use that exception,
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you have to use the as keyword,
did you remember that?
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If not, don't fret,
we only did that one time in this course.
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The more you practice,
the more you will be able to recall it.
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So, you have to use the as keyword,
so as err.
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So, no, we ran into an issue, I'm gonna
go ahead and drop our message there,
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then I'll, was that a period,
space, please try again.
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And then we will format that
with our error, format, err.
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There we go.
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Okay, I'm going to break out of this,
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I'm going to clear this and
run it one more time.
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What is your name?
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It is Sir Robin.
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I would like 10,000, no we've ran into an
issue, there only 100 tickets remaining,
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please try again,
there are 100 tickets remaining.
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Awesome, so I think we can
move this final ticket over.
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How good does that feel?
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I am gonna go run a demo for
Money Python and see what they think,
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they're gonna love it,
I know they're gonna love it
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