WEBVTT

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so just when you thought we were able to actually move on from

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blackjack but actually we got a little bit
more to talk about and we are gonna use

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blackjack as this example now this is
specifically talking about underscores

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in variable names now we've had a look
at a few of the modules in the Python Standard

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library in previous lectures
and we jumped into the source to check

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out how to use a few things and when we did
that you may have noticed that many

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object names start or end with
underscores or even start and end

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underscores and you might be wondering
what that's all about and obviously you can

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see on line 3 we are using underscore underscore name underscore underscore now

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we've discussed ending a name with a
single underscore when we looked at the

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spin box widget in the Tkinter video
and you can set the range of values

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that appears in the spin box using the to and from
underscore arguments but because from

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is a Python keyword we used from
underscore rather than from now a convention

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is to use a single underscore after a name
if it would otherwise conflict with the

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name that's built into Python so
reserved a keyword in other words it's

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probably better to use a different name
of course but if the name that Python

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already uses makes sense for your
particular use then probably adding

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an underscore to the end of the name is the
way to go

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you can do that quite easily so in the
case of specifying a range of values

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from a spin box widget to and from a
probably the best names to use so the

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creators of the Tkinter module uses
convention adding an underscore in this

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case to the from keyword so that's the
easy one out of the way now for other uses of

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underscores around

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names now unlike many other languages
Python has actually got no concept of

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private or protected variables that
means that it's possible to do things

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with a module that you probably
shouldn't do so let's have a look at a few

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ways we can miss with our blackjack
module so we've got the project open from

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the previous video we made the changes
in the previous video to it so we are able to

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import it as a module and the project
should now have 2 Python files as

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you can see on the screen now blackjack.py and import_test.py so

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it's useful to be able to call the play
function as you can see me doing on line

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4 and also using the cards list to
create decks for other games is also

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extremely useful but beyond that though
their is not much else in the

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Blackjack module that really makes any
sense to use once it's actually been

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imported now there is a score_hand
function but that's really very specific

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to blackjack and couldn't be used to
decide which of 2 poker hands are better for

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argument sake because it's a completely
different algorithm

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now in Java for example only the play function
and cards list would be made available to

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other programs everything else would
have been marked private so that it was

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hidden and then it wouldn't actually
show but as I said Python doesn't

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have any or doesn't have private objects
and that means that we can do silly

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things after importing the blackjack module so we are going to start by calling the deal_cards

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function just to show you that it can be
done and not suggesting it's actually

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a wise thing to do so come down here if we wanted to we
could do something like....

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....and so let's run that just to

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confirm that it does actually work so right click this and click run to make sure

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we're running import_test and not
blackjack so do that we run it and tab over and

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have a look and obviously we've got two
cards now the dealer has actually got 2

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cards although at this stage due to the way
we designed it only the second one

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is actually included in the dealer score and as a result that does look really

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strange and that is because we've got a
total score of 13 for the dealer

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but only the second one is calculated
again this is an ideal basically

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giving access to areas of the game that
we wouldn't ordinarily want someone

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who's imported this blackjack module to be able to
access our code that way because it's

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sort of breaking the game as such this is
compounded by the fact there's really

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nothing to indicate that we shouldn't be
calling this function so from that

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perspective of someone who's imported
the blackjack module and is wanting to use

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it their's no clear indication that you know
maybe I shouldn't be actually calling

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this method here and obviously by
calling and then by trial and error

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and running through and testing it and seeing that
it doesn't work they will eventually figure

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it out but there's no easy way to know ahead of time if

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that's the case by convention starting a name with an underscore indicates

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that it should be treated as protected which means it's not intended to be

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used outside the modules existence so
it's really a standard in Python now theirs a

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little bit more to that when using
classes and we'll talk about that in the

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next section when we start object oriented
programming but if we go back into the

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blackjack module

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we can actually refactor the deal_card
function so that it's called _deal_

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card so we go back their go to our deal card and if I refactor that right

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click it refactor rename and put an underscore in
front of it and refactor it...that should

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have changed everywhere even here

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back in the import_test.py but notice
what happened now IntelliJ has actually come

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up and help us now its access to a
protected member_deal_

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card of a module so we are now
correctly getting a warning that this is

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a protected member in so much as far as Python is actually concerned so their is still

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nothing preventing the access just wanna
make that clear we can still run it and

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if we do run it the game still actually work so unlike Java or another programming language

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where once we set it to like private or
protected we can actually restrict the

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uses here we can't do that so it's still
gonna work but you do get a warning

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that it's quite prominent its not just a little warning on the right hand side its actually

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underscoring

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you know the little underscore on the actual method name we are trying to access which will hopefully

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get you to look at and think what have I done wrong here
so if you find yourself using an object

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whos name starts with an underscore now
bear in mind that you probably shouldn't

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be doing that the developer of the module
has defined it that way for you probably not

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to access it that way of course if the
object is in your own code then you

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probably know what you doing then its ok
but just be very careful if it existed

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in a module that you didn't actually write

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yourself and as you can see IntelliJ is
aware of this convention and provides us

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with this very useful warning so that at
least we are aware of what

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we're actually doing so just close it
down because there is actually another

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implication of using underscored at the
start of a name so if your using

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alternate way of importing blackjack
anything whose name starts with an

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underscore is not actually imported
now we've recommended before that you

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don't do an import* from a module
because all of the names from the

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imported modules then have become part of your
modules namespace and it's then very

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easy to lose track of what names belong
to what because the way we imported blackjack

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we need to use blackjack. to access any
of the Blackjack objects which makes it

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very obvious that we're actually dealing
with code that's part of blackjack

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and you can see on lines 5 and 6 we are actually inputting the . their to

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access that blackjack specific functionality so we can now

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look at the difference in more detail so
what we are gonna do is comment out all this

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code for now just leave the import in we are gonna
have a look at what's in global so I want

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to list all the names separately rather
than in one long line that we get if

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we just used print globals which have
used previously so gonna create a

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loop here so...

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...so if we run this but we get
an error straightaway so x is meant to

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iterate through all the keys in the
dictionary but as soon as we've used X the

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dictionary changes and Python gives
us an error so this code does not actually work

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because obviously we knew very well that
we defined that has actually altered the

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dictionary itself so we can't actually access the
the globals that why so one quick way

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around this is to take a copy of the
globals dictionary then iterate through that

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so that means we won't see our variable X
in that list but as a quick way to make a

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copy is to use the sorted function then
the actual code has the added advantage

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of producing a sorted list so let's go
ahead and do that so.....

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....

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.....so that way

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there's nothing being added to the
dictionary when we go to iterate

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through G and we should find that
it actually works without an error so

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if we run this this time it actually has
worked so the list itself is pretty

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small and a part from a few things underscore before and after their names

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the only object that's present in here
is blackjack which of course we

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imported on line 1 but let's just change
the import and see what happens if we

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added a start so...

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.....

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...we do it that way and we run it....you can see we got a completely

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different result but all our functions
as you can see and the variables are

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showing up basically everything from the
blackjacks module namespace now appears

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in our modules namespace so if we
created an object called cards for

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example then the blackjack modules card
would no longer be available so I said

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that everything from the blackjacks
module namespace is imported but to be

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precise that's not strictly true for
example there's no assign there if you have a

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look at the screen of our _deal_card function and if we

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try and use it now we actually get an error so if we try and do that....and we got an error their unresolved

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reference so the Python import
mechanism does actually take note of

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this convention and it won't import any
objects that start with an underscore

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when you import star now when we
import blackjack the objects from the

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blackjack module aren't imported separately so
everything even names beginning with

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underscores are available by the prefixing them with blackjack. so we

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put the import back as to what it was like
before

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....that's not gonna work
because it's an unresolved reference but we

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go back and put blackjack in front of it

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to tell Python that it's the
Blackjack module we are dealing with it and

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for the frame....the error goes away and we are good to go we still get a warning here

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from IntelliJ telling us that its accessing a protected member but it does work again now lets get rid

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of this code......our app is running again all being with two cards again with

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only one being scored so using a double
underscores at the start of a name invokes

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Python's name mangling rules so this
convention really exists to prevent name

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clashes when subclassing objects and
again we'll look at this in more detail

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in the next section when we start dealing with
object oriented programming but in a

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blackjack module two underscore would
really serve no useful purpose but of

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course anything that starts with two underscores
automatically also starts with one so

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import* wouldn't actually import
them now the final underscore naming

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convention to discuss here is names that
start and end with two underscores now

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basically things that
you shouldn't be changing now we've seen just

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deleted it __name__ and use it to restrict the code

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that was executed when our module was imported so it should be obvious from

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the example that changing the value of
the __name__ isn't a

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good idea so of course I will just go ahed and do just that
now to show you what happens the

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same is generally true of anything
that's name with double underscores that

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at the start and end they can be useful to use but they should never be change so we are gonna go ahead

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now to our code and make sure that it's set correctly and I'll actually get rid of this code their....so now we've got our import we got our blackjack deal_card and blackjack.py

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so we are ready to go with that so we tested
that will just test one more time to make

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sure that it does work obviously our app is working nicely so that is good and of course we're

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getting two cards because we're using
line 3 calling that method when we

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shouldn't be doing that so next what we
gonna do is we are going to break things by

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changing the value of name the __name__ in blackjack.py so we are going to their and close this run window down

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just below the play method what I'm
going to do is change this variables so...

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....

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so obviously this is something you
shouldn't be doing good just making the

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point that these two underscores at the start and end of the variable names is something

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is Python way of saying you should never
change this but we are just doing this to see what

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happens

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if we run our import_test and have a look so you can see that the

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game starts up fine but notice that the
dealers has got only one card so the code

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the import_test hasn't yet
executed and that this is the original

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effect that we saw when we first imported
blackjack because __name__

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has been set to main or the __name__

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I'm talking about this code

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code we added to line 125 because we change that
the test that restricts code execution

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on import is no longer valid and the
code runs when we import the module

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so then if we actually get back to our
code and close this we then get an error because

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by closing the window we've destroyed all
the TKinter objects so that nothing

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works and the game cannot be played by calling
the play function so Python doesn't prevent you

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as such from modify these variables but
if you do so then the results are

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undefined and you can run into all sorts
of problems like I just introduced

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to you the only thing you can be reasonably sure of is that their is something that is going to break so before

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moving on i'm just gonna remove this offending line again because that shouldn't be

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in their deleted that like so close this run
window so we just about done here and what I'm going to

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leave you with now is one more use of
the underscore that will quickly go

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through so a variable that's just name_ with nothing else indicates

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a throwaway value so underscore by
itself is a valid variable name and

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rather than

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thinking of a name for something that's
not gonna be used the convention is to

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call it either that underscore or underscore
underscore

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so examples of things that you might have
to access but are not going to use include

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tuples we want to use some of the
values of a tuple but not all of them

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let's just write a simple code I'm just
going to comment out blackjack for now and put it below that

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and will put.....

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....

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...

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.....so as we saw in the tuples
video at the start of this course you can

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assign all the values of a tuple to a
variables using a single assignment that

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I have actually done here on line 8
but this code isn't really

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interested in my age just my name and
country and so I've put an underscore

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where the age would actually be now we could actually add another line to print the age if we wanted to

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so....and that would
still quite happily work run it to make sure it

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does work

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Tim Australia and 24 so it is working
but the point here is that it's just a

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way for you to name something as a
throwaway value that you know that you

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are not gonna be using for the rest of your programs
obviously in the case of the age which

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was just a second part their that is set to 24
currently if we actually gonna use the value then

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you probably give it a better name than the
underscore so the single underscore

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or 2 underscores as variable names are
used when you would not be doing

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anything else with the value but you
have to specify the variable ok so

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that's it I'm gonna finish here now and I'll see
you in the next video