[MUSIC]
Hi, I'm John.
Welcome to my first video for this course.
So, we're going to be looking at a bunch
of examples.
First on errors.
When you get an error message, how do
you understand what it means and fix the
problem.
Second we're going to look at a few
probing tips
that should help you when you're writing
your own code.
So here's our first example.
Let's run it and see what happens.
It starts to print out this message about
the volume of a cube,
but then has a name error saying that
volume is not defined, kay.
It highlights the problem line here, where
in fact we
are using the function named volume to do
our computation.
But where did I define volume?
Well, I didn't.
I defined volume cube, kay.
here I should be calling volume cube, so
let's fix that, kay.
So, now let's run it again and make sure.
Oh, whoops!
We have another name error.
This time, I used the name Sidde here in
this line instead of the variable named
side, side.
Kay, so a typo here.
Kay, so running it finally it works.
So, notice that we get a name error when
we misspell either a
function name or a variable name here and
don't have a definition of that name.
Alright, let's see what happens with this
next example.
We'll run it and well, once again we get
a name error here, name random is not
defined.
So well, we're trying to use
random.randrange to roll a die.
Hmm, what's the problem here?
well, here let's go to the documentation,
kay.
And we can scroll down here to the random
module, and it tells us well, yes
random.randrange is right.
But whenever we need to use it well, yeah,
import
random, we forgot that and it tells us up
here.
You know, whenever we're using anything
from this module, import random.
We forgot to do that in our code here so
let's add that.
Yeah, we didn't provide that
definition of random that it's a module
here, so
let's try this again, running it and it
works fine.
It, it, rolls our dice and each time we
get a different value for our pair of die.
In this example we want to compute the
volume of the sphere.
Let's see what happens when we run it.
We get a attribute error.
Module object has no attribute pi.
Hmm, well we're using math.pi and I'm
importing math correctly.
So what's the problem?
I thought pi was defined.
Let's re-, let's go to our docs, and we've
got a little delay on load right now.
let's search for pi and there we go.
There's math.pi, but it's lower case,
okay.
All of our names are case-sensitive, so I
misspelled this, and any time
you misspell the part after the period,
that's going to give you an attribute
error.
So changing that and rerunning, it now
works.
In this example we want to calculate the
area of a triangle so, well.
We get a type error, area triangles takes
exactly two arguments but one was given.
So let's look at our function call which
is here, area triangle b.
Well, in fact, I did give it one argument.
instead of the two that it's expecting I
forgot
to pass an h, so let's add that.
Okay, and re-run.
[SOUND].
I get another type error.
Oh, hmm?
But it, at least it's different.
we fixed that one problem.
Type error can't multiply sequence by non
ends of type number, huh?
Alright, let's see what it's complaining
about.
Okay, so I am doing a multiplication like
it's talked about.
As we need to in this formula.
But, what's going on?
So, let's look up at, at what's actually
being printed out, and it says,
the area of the triangle with base five
and height two plus two is.
Height should be four.
Why isn't it adding two plus two there to
get four?
Well, the problem is we have strings here
not numeric expressions.
This is
simply the string two space plu- two space
plus space two.
It's not a mathematical expression
resulting in four.
So that's why this is resulting in
multiplying a
sequence because, well, it's trying to
multiply a sequence base,
which is a string and a string is a
kind of a sequence by something that's not
an integer.
Well, height here is also not an integer.
Turns out you can actually multiply
strings from an
integer, but that's a whole separate
thing, separate operation.
Kay, so let's just get rid of the double
quotes here.
Turn these into mathematical expressions
and rerun and we get what we want.
So, next here we have a little function
that's supposed to simply check whether
we're giving
it Mary, and if so it gives us a message,
that yes it has Mary, otherwise no.
Okay, so let's try running this, 'kay.
Syntax error, bad input on line one.
[SOUND].
Quote, you know, and then quote down here
on the next line.
Kay, so it tells us there's a syntax error
where it is roughly, on what line.
Okay, and the fact that, well, it's trying
to tell us here that it's
it's at the end of the line, where, where
it's encountering a new line.
Kay, and so here it's just kind of giving
us a new line there.
Kay, so that is hinting that
it must be at the end of the line here.
And in fact we forgot to put the colon on
here.
Remember, every function header begins or,
ends with a colon.
Just like a every if here ends with a
colon, else ends with a
colon, anytime that you're going to start
a block of code, kay, so, fix that.
Let's see it run now.
Oh, another syntax error.
Bad input on line two now.
Okay, we have if x is equal to Mary
there's something wrong with the equals.
What's wrong with the equals, hmm?
Let's go back to the documentation here.
And we could, let's try comparing things.
Let's see, where would I find something
about comparing?
So, compare, oh, comparison operators
there.
let's see.
Oh, the comparison here is double equals
not single
equals kay, very common mistake.
Single equals is assigning a variable,
double equals is comparing two things.
Okay, so
let's try this again here.
Oh, name error, name Mary is not defined.
Oh, kay, yeah, down here I gave it.
Well, Mary is not defined, I, I meant to
pass in just a string.
Okay?
Here, note that note the color, okay?
Well this is kind of useful here.
So, in this code here where we have
some strings, note the strings are
color-coded red.
And this is not color-coded red.
It's just black like any other name.
And well, notice over here we actually
have purple stuff too, for keywords.
Kay.
Notice when you're typing,
whether things are turning up the color
that you wanted.
Kay, and so here, basically just another
kind of syntax error.
I meant to give a, meant to give it a, a
string instead of a variable name.
So let's see if I'm good now.
Oh, good.
You know, it found Mary and then Fred,
well, is clearly not Mary.
So, fixed it.
So let's turn to the issues of code style
and readability now.
So, just trust me and that this code
actually works.
So, we have some code, what does it do?
Def area a, b, c and some math here.
Not really clear, you might not know this
formula.
You know, if somebody else is looking at
this
code after you've written it, they don't
know what you meant probably.
In fact if you're looking at this a month
after you wrote it, you might not remember
either.
So, what should we have done?
We need to provide more information to
actually explain what's going on here.
The first thing is what is this supposed
to be doing?
Kay, let's make that clear.
So, this is actually supposed to calculate
the area of a triangle.
Kay, so let's give it a more useful name.
Area of a triangle and I'm going to
call it area triangle_sss to be sure that
it's taking three sides, okay?
And so well, no matter how long a name
should, can be you don't want to make it
too long, it gets just kind of unwieldy.
You can provide
more information in what Python calls a
documentation string, or a doc string.
And in Python, that traditionally goes
right here, right
after the function header, and it's put in
a string.
Kay?
Even though this is just a string, we
traditionally put it in three quotes,
three double quotes.
what the three double quotes allows is
that, it allows a string
to go over multiple lines.
Kay?
Now, once you go into a document string,
you should talk
about what are the inputs what's the
output and how those relate.
So, here we could say for example,
that this returns the area of a triangle
given the lengths of its three sides.
Kay?
Now, the documentation string tells you
what it does, it
does not tell you how it actually does
that, kay?
So we've got some weird math here.
You might recognize it, you might not,
kay?
It's actually a standard formula known as
Heron's formula.
Kay.
So, I'm going to write that down.
You know, somebody can look up Heron's
formula and
say aha, you actually typed in Heron's
formula correctly.
Kay?
Well, I'm going to put this in a comment.
A comment starts with one of those hash
lines or pound signs.
Kay.
And note that I'm not putting this
in the documentation string, kay, this is
different.
This is telling somebody, how I'm
calculating
this, as opposed to what I'm actually
calculating.
Kay, so that's already better.
Somebody could understand,
given the English and the better name,
what's going on.
But still let's, let's improve this here.
A, b, c, you know, those are kind of poor
names.
What are those supposed to be?
They're supposed to be sides, but let's
make this clear.
Let's rename them side one.
And I'm going to add in some space that is
for readability, too.
Side two and side three and be
sure to do that consistently.
[SOUND].
Alright, and similarly S, well that,
that's kind of a weird name.
What is that?
Well if you add up the three sides of
a triangle and divide it by two, what is
that?
Kay, adding up the three sides, that's the
perimeter.
Divide by two, that's half of a perimeter.
In other words
it's the semi-perimeter.
Kay, so that's why it's traditionally
called s.
Let's actually give it a longer name
called semi_perin.
Let, you know, more descriptive.
Add a couple more spaces
in here for readability and rename
consistently here, kay.
Be sure of the consistency.
[SOUND].
Now one thing you can see that's not
very readable is now I have a really
long piece of code, a really long
line.
Well you can break that up,
kay.
Notice the indentation.
So, I have the one formula within the
square
root that's broken up on four lines so
it's
more readable and, and in fact they stack
on
top of each other, four things that I'm
multiplying.
So, this is definitely a much more
readable version
and somebody could read this and
understand what's going on.
So, this little program here is supposed
to tell us,
you know, what is the favorite thing of
our various instructors.
So, Joe loves games, that's this course.
Scott loves ties, as you should be seeing
from his videos.
And I love the outdoors but, well, I'm
here in front of a computer all the time.
So, well, let's run it.
And, well, it reports that I love the
outdoors.
But, hm, Jeannie, my wife,
who's not an instructor and none.
Where's that none coming from?
Well, first of all you know, note that
because Jeannie is not
one of these options, it doesn't meet one
of these branches here.
It just falls through and at the end of
the function there's nothing left to do,
so it just returns a special value called
none which signifies that there's
no value there.
Kay?
Now that seems kind of strange, and in
particular in a larger program, if this
was just
a piece of some larger thing you'd
probably have
a hard time tracking down where the
problem was.
Kay, so we'd like to have some way of
saying, hey something's wrong
here, you gave me a valid, an invalid
input that I wasn't expecting.
and so here is one really simple
way of doing it.
Just make sure that each of your ifs has
an else case, kay?
Else, kay, some other inner
value of instructors here let's
print out a message you know,
favorites, saw invalid instructor.
and here's our value Jeannie.
Whoops, instructor.
And so we can run that, and it gives us
this message saying, hey, this function
saw an invalid input, you know, now I know
where to look, and where the problem was.