WEBVTT

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so I mentioned at the end of the
previous video the parabola function

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itself on line 7 isn't really doing all
it could do though because the main

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program needs still needs to loop through
on line 34 where the loop is starting on line 34

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and go through all the values it
returns in order to print a parabola so

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I think it'd be more useful if the
function plotted the points and all we

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had to do was tell it how big the
parabola should be and then give it a

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canvas to draw on now before we get
into that though I just want to talk about

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some of the warnings that are on the
screen and remember these warnings here on the

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right hand side let's just fix a couple
of these up so the first one is at least

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two spaces so I made a mistake we need 2 spaces when we are putting a comment

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in line at the end of a line so that
should fix that one up the second one

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camel case variable import as lower case
that's because we're doing import with a

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capital T for Tkinter and s the lowercase Tkinter
for Pythont 2 so that is ok we

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don't need to fix that but these other ones
here are all the same shadow name x show named

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Shadow named canvas the errors is the same basically it's a different name but

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shadow name from out of scope is the error
now what it is is one of the variables

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in our function shadows of variable
with the same name in the main program

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so the first one actually reads if you go back here and have a look actually we will do a better

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example come down here to the canvas so shadow name canvas from outer scope

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inspection that detects shadowing names defining outer scope so when they talk about

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the outer scope what they mean is the
main program where we created a variable

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called canvas and you can see that we
did get on line 29 the errors actually

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coming up talking about the canvas
variable that is defined on line 12 as a

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parameter to the draw_axis function and that's because when I

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gave the parameter to the draw_axis function and name it canvas it the function

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then lost the ability to see the canvas
variable in the main program that's

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because they're both the same name so all
references to canvas within the draw_axis

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function

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mean that the perimeter referred to the
parameter that was passed in and not the

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canvas variable from the main part of
the program even thought they are called the same thing

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that's actually the reason why its
confusing so it's really important to

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understand this variable scope as this is what it is and as

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functions are one of the few big things
in Python that actually introduce new scope

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it's worth spending a bit of time here
just to understand that so you are aware

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of how this actually works so to explain
the difference between the canvas that

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is local to the draw_axis function and
the one thats global what we gotta do is

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create another canvas and then side those both so that they can each occupies half

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of the window now our original canvas

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will need its width changed to 2320 and this
is one on line 29 and the reason for that is

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because we gotta put two canvases where
there was one before the same space so

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if you're using a larger width and height

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you will just basically divide it by 2 in this
case we are going to make it 320 that's the first one

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and we are adding it to grid which is correct but then we are gonna add the
second variable.....

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.....

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....the difference
will be on the next line we're going to

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add it into the next column so of course it
appears to the right hand side so...

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.....

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...and we got a draw_axis function on line 35 which
is correct but on line

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36 we're going to add the draw_axis for the other side so.......

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....so just confirming that if we actually

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run that to see that it does work
 and you could see it basically got

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two canvases now we've got to the first
canvas which were drawing on the half

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basically left hand side of the screen
and our parabola drawing just on

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that and we also got this canvas on the
right hand side as well

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and we are drawing the axis to that so in
the past in the previous program of course

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it was just one occupying the entire 640
by 480 for our window so that's working

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and actually what I didn't do what I might do now is I am going to come back here just to make it a bit more obvious

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and then I'm gonna put....

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....and that will make it clearer because we
got a blue background as you can

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probably guess that will do now its a little bit
more obvious now that we got this new

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canvas which we actually added which is now got a blue
background ok so each time it's called

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it's own variable called canvas is assigned the canvas object that is pass to

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the parameter in the call so here
basically on line 35 passes up a

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passes the canvas object
and the passes on to line 12 to the draw_axis

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function the that calls that canvasses as well
and likewise for canvas 2 even though we

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referred to it as canvas 2 its
referred to as canvas for the function

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name and it's going to be using that name when
it's processing that for also for canvas 2

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now we can see that a little bit better by
printing out some details of the canvas

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objects so there's a function that shows
the representation of the object called

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repr r-e-p-r so let's just put that
in and you can actually just sort of

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print out some details about an object
in a format suitable for printing

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so i can type.......and that will just printed

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it out each time it's called it's going to
print out the basically the memory reserved

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so if we just run that to confirm it is actually working you
notice now that we've actually got this

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little print out on the screen now and you can see here the number at the end of each

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part of this tkinter.Canvas object print out is meant to represent the memory

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location where that particular object
was created in memory so obviously there

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we got a different number in both cases in
which means that they're obviously

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separate objects which is correct but
the other thing is just out of interest

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if we stop this close it and run again you will note those
numbers will be different

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basically different each time you run
the program different in that these will be

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different to each other because they are
separate objects but also the individual

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numbers allocated to each object is also
different because the computer every

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time you run it is a assigning its using making a
decision where to put these in memory and

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that changes each time you run it and no doubt these numbers are likely to be

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a bit different when you run it on your computer as well
that's good so we can see that their

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separate objects

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but what is also cool is we can print
local variables

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so Python has this ability called using the
locals function to print out all the

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local variables for a method so what we can do is make a change to that so lets put this at the end of

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line 18 so type....and that is the locals is the method that enables a printout of

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all the variables that have been defined
for that particular function obviously

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has to be the code has to be in a
function too be called its

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going to just basically look at all the
variables in that function so if we run

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that you can see their its showing us that we got the canvas object and note that the numbers here

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so for the first time we call it canvas
represents the same variable

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canvas this first one here as we pointed that out and then
also its got a y location

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y_origin and x_origin and that is the variables
that we are creating on line 14 and 15 but note

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that we are getting consistency here we got
the number their corresponding to that

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number the first time it's called and the
second time that number corresponds to

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that one so we can see that it actually
is working so it can be useful tool when

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you're debugging to have the locals
function and also the representation code

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the repr function as well so you think
at this point ok so everything's working fine

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so why is IntelliJ actually coming up
with these warnings what is the problem

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well firstly with a small program like
this where we can pretty much see

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everything at once on the screen is not
really that big a deal I mean all the

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codes there on the screen we can access
it and you probably not gonna get into

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difficulty but chances are that you
gonna be ending up writing much larger

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programs and you can find it can cause
unexpected problems if we make a mistake

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somewhere pretty much I can guarantee that will happen on

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some point on your programming journey that you make a
mistake so what I'm going to do is

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simulate one possible mistakes in the
way to simulate that is to rename the

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parameter canvas and to call a page so
I'm talking about the parameter here

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on line 12 so we are gonna change that to page you
can see their and we need to make a few

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other changes we need to call this
now page.update so.....

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and as I'm going to be doing this you might be
thinking well Tim why aren't you using the

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refactor facility

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well it's slightly contrived example
here because obviously that would

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automatically rename all occurrences
for us but it's possible that you might

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be making a quick change using some
other editor as I'm doing it manually

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to simulate a potential problem so lets just assume I did that and I remembered most

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of these so I'll change that one as well

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.....

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......so if I actually run this having knowing
that I forgot to make the change on line

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18 and you notice we've got a different
output here we've got no y axis

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showing in the right hand canvas so you
might be looking at that thinking well

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why that suddenly stopped working

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you know if you made this quick change like I have and you neglected to make that change as I did

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on line 18 and what I need to do is run it again so we can see now that with the output from

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the locals function that the variables
is called page so on line 18 the yaxis is

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being drawn on the canvas from the main
program so that one has its axis drawn

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twice effectively so I basically we can see
what's happening their the canvas

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variable that's been referred to their on line 18 is actually the canvass

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variable that was defined in the main part of the

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program and not from this function now again I did say this was a bit contrived

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but if I originally called the parameter
graph instead of canvas for arguments sake so if we

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comeback and do that lets

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presume that it was called graph......

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and we actually run this clearly we get a different error
altogether and the reason we've now got

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an error is it Python notice that ok this
variable graph doesn't exist we are not

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shadowing the canvas variable from the
main program anymore and consequently

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we've got you know we know that there's
a problem we can actually fix it this is

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probably not only crops up if you try
to use a name the same name in your

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parameter as your using for your variable that is
defined in the main part of the program so

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although python did mind that local main variables have the same name it can

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be a source error and that's one reason why
IntelliJ is warning you about that for

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these types of errors that you might run
into and it's probably a bad idea to

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have the same names but with that said
there's also another reason why

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shadowing is a bad idea in Python and
will have a look at that a little bit

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later so this example has also hopefully
demonstrated the scope of the variables

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so draw axis has its own local
variable called canvas

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that is now called page and in the main program there
were two canvases called canvas and canvas 2

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but inside the draw_axis function we can
and did actually refer to the

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canvas from the main program but if we
now we try to refer to pages in main program

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it won't work so it doesn't go 2 ways so firstly I'll fix this Graph put that back to page so

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clearly in the draw_axis function
we're able to refer to these variables

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defined in the main part of the program
of the Python program but we can't do

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things in other ways in other words we can't
come back here for example and put print

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...if you try to do that you can already see you got an error and if we try to run it name page is not defined and that is

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because we're a variable is visible it can be
used is called scope and that's really

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all there is to understanding the scope in Python now their is some other some other things we

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can do to specify the scope of a variable
but we'll look at that later but for now

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just remember that a function can use
the variables from the main program but

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the main program cannot see variables
that are local to the function so I'm gonna

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end the video here actually what I'll do before I end I'm gonna delete line 40 so

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that we haven't got any code errors their

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and will continue on with this in the
next video