WEBVTT

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so continuing on from the last video so
global keyword tells Python to

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look for a variable in the global scope
which is the modules name space but the

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non local keyword tells it to look
for that variable in the enclosing

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scopes and you could see what we've done
differently to other videos is

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we've got an embedded function here that we
didn't have at the blackjack program so

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that's why on line 7 the global keyword wasn't appropriate so again the non local

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keyword if that is telling Python to
look for the variable in the enclosing

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scope so non local won't check the
global name space though so a

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non local variable must exist in the enclosing
scope but not be global otherwise if it

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was global of course you would use the keyword
global to do that another thing to know

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about the non local variables is that they
must exist in the enclosing scope

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they can't be created in other words by the non local keywords so if we change this to try and put a declaration

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in their we get an error...end of statement expected
because it's looking for us just to define

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the variable to say that it's a non-local
variable but it has to actually exist as

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I mentioned in the outer scope or in the
enclosing scope to be more precise now

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although we didn't do this we could have
created our global variables for

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blackjack in the new_game method and they would have been

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available in the main program but it's
not considered good style to that and

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can lead to difficulty debugging the programs
but it's there and it is actually

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allowed but the same is not true for non local variables and that's because Python

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doesn't know which enclosing scope the should have existed and has really has no

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way to know whether to create them so
they must already be present in an

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enclosing scope or you get an error for that reason so at this point its a

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good time to point out that the only
things that create scope in Python are

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modules functions and classes now I've already
talked about variables in a module

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being defined in a while for loop and
then and remaining available to the rest

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of the program but that's because they
become part of the global name space now

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that can seem odd if you come from Java
for example but the idea that functions

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introduced a new scope will be familiar

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know when you define a function Python creates a new name space for it and

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any variables are part of the local scope and to that end we actually printed out the local

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variables using the locals function in
the blackjack game a few videos ago so non

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local tells python to look for
variables in an enclosing function and

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if it doesn't find it there to check in a
functioning closing that and so on

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it will not though check the global name space
as I mentioned so if you remember

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that it doesn't include globals then non local is a good name for it variables that are not

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local to the current scope ok so
hopefully that made a lot of sense

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earlier implied that functions can be
nested inside functions and themselves

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are themselves nested in another
function its quite rare to have

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more than one level of nesting in any
code that did so would have to be quite

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complicated so I use a variation of the
spam function in the tutorial to

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demonstrate a deeper level of nesting
and bearing in mind its probably not

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something you have to do very often but
the functions would be very similar and

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just returned different amounts of spam

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in this case but that will let us focus on
the scope issues rather than getting

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bogged down in complicated code so we are
gonna create a new Python file called

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spam.py and we are going to enter the
following code in it and just do a test

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on this and have a look at these functions so...

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....

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....

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....

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.....so as you can see

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here the function spam one contains
another function called spam 2 and

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that's spam 2 function contains a third
function called spam 3 and if we run this

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you can see  that spam 1 calls the spam 2 function and then adds its
result on to spam 1's text and

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spam 2 in turn adds the result of the spam
3 function on its text and

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subsequently we get a lot of spam
now to get a clearer idea of what's

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happening here we can print out the locals
dictionary from inside each of the

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functions I'm going to use a format
string so that we can add a label

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before each dictionary to be sure where
it came from so let's go ahead and do

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that i'm gonna start in spam 3....

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....

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....

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so if we run this again now I've done the same thing in my haste here

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so that should have been tabbed over
my bad their when I was adding the new lines so

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try that again now we can see now and you can see that

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it's showing which names are local to each function and we can see that for example

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the function spam 3 just contains
the string variable z but spam 2

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contains the string variable called y
you can see it here nearly off the screen but it also contains

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a function as you can see there

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called spam 3 you can see that reference their and also spam 1 has got a local variable X

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and a function called spam 2 as you can
see here so within spam 1

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itself we can't refer to the
variables y&z because variables in an

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inner scope aren't available to the outer scope but however inside spam 2 we can

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refer to x and inside spam 3 we can
refer to both X&Y

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so an inner scope has access to the variables defined in an outer scope and we can use the

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outer variables and spam 2 spam 3 lets do that by making small change to the code so

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close it down lets have a look on what its gonna look like so if we change this now and I'll make that even....

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....

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and run this to make sure it works so you could see that is working ok

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and because we're not assigning to X&Y in
the inner functions the names don't shadow

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the variable in the outer scope and as a
result the values are then available for

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use now I'm gonna demonstrate scope and
it's not necessary sensible thing

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to do here but it will demonstrate what
we're trying to get across to you and

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the reason is...

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lines 16 and 17

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cam be combined into a single expression
and when I change them lets have a look

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and see what happens so this here I'm gonna make...

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now the mini challenge here is before actually
running this program can you work at

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what's going to happen and why its going
to happen because I only done a simple thing

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as you can see I've merged lines 16 and 17 to make it
one line but I want you to see if you can

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figure out what's going to happen and
why so pause the video now and have a think about it and when

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your ready to continue come back and we'll go
through it

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ok so hopefully you have a look at it
you might be actually curious if you didn't

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figure it out so let's talk about what
actually happens now the change I made was

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quite simple and it might appear at
face value to be a simple optimization

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but in actual fact that simple
optimization breaks the code and without

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starting the spam 2 function it wasn't obvious that the code was gonna break

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so lets run it first to confirm that there
is a problem and you can see we get

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these weird errors so three variable X is
referenced before assignment in closing

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scope

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we got errors here on line 2016 and 11

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so what actually happened is that spam
2 the function attempts to use the

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value of x but spent spam 2 is called during the
initial assignment to X so x doesn't

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yet have a value as a result we get the
error free variable X

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this down here on the bottom
free variable x referenced before

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assignment in closing scope so theirs
actually 3 lessons to learn from the

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simple change I'm gonna move the it their so we can look at the source code at the same time while we are talking about it so

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the three things are wherever possible
try to write functions so that they only

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use local variables and parameters only
access global and non-local variables

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when it's absolutely necessary if you
can see an unexpected code such as our

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use of 2 lines to assign x and even also to assign y on line 11 and 12 think carefully before

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changing them because it could be that
the programmer who wrote them was having a

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bad day but also their could be a very
valid reason why they used 2 lines

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when one would appear to do so no matter
how trivial a change do you think you've

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made test the program thoroughly to make
sure you haven't broken anything that's

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often simple changes like that that will
actually break code and major changes

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will probably get through testing anyway
but very simple modifications can get

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overlooked and this will be a case of
the change that I've made on the 16 and 17 to

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merge them into one line would be
something with that would often

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get overlooked and actually there's a fourth
lesson here as well and that is that if your write unexpected

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code such as our use of 2 lines we assigning X&Y make sure you include a

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comment to explain why so the next
person who works on your code doesn't do

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what you've just done so what we do is undo the change put it back to how it was

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before and we come back and say....

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.....and on lines 11 and 12 we would

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say something like....

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...you will actually be surprised how often that next person is

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you so coming back to the code perhaps
have looked at the program you have written for a

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number of days weeks months or even years so the person that you are writing

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this comments for could ultimately be used and I just put in an extra space here should have bee watching the

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warnings to keep the compiler happy so yeah the person could actually be you

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that you're trying to help out here and you can quickly really forget why you did something

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in a certain way so using comments like
this to explain anything you do that's

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not obvious is very very highly
recommended now before I finish this

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section out theirs two more things to look
at the first is very simple

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their at the module level the local scope is
the same as the global scope so we can

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see that by printing both out at the end of
the program so lets add these couples of lines of output

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to just check that so...and if you run that

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and if we scroll across you should find that the basically
the two outputs from locals to globals

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is giving us the identical output here you can
see this looking pretty well

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identical right through to the end
everything is basically identical that's

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for those last two lines of output which
correspond to the output from line 22 to 23

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so that confirms that in the module scope locals and globals are the same

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second thing though is a bit odd and
you might be wondering about it if

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you paid careful attention to the
program's output since I added the non

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local variables so lets have a look at that again so spam 3 original
had a single variable called z but

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now you can see up here it also has
y as local and spam 2 you see that has

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now got an x its got the x as local as
well now this is very odd it seems to go

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against everything I have just discussed
about non local variables but in fact

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the explanation behavior of non local
variables is perfectly correct so

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there's nothing to worry about what we are
seeing is a result of the way the locals

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function works and some optimization of
the part of Python and that's because whether

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you use a non-local variable in the way
we've done python adds it to the local

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name space so that it doesn't have to
go hunting through all the enclosing

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scopes every time you use it so it
still referring to the non local variable

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its just a reference it in locals to make everything run faster and it might be worth your while

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having a look at the documentation for the locals function as well I'm just gonna bring that up on the screen

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theirs the link on the screen as you can see up there so we go to that

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theirs locals so a free variable is a variable that's
used in a scope where it has not been

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defined which is exactly the same thing
as a non-local variable when working

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with the scope of variables it is helpful
to remember the analogy

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LEGB and that stands for local enclosing
global built-ins and list the order to

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which Python searches for names so again
its local enclosing global and built-ins so

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closing refers to non-local or free
variables that I've discussed in this

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lecture now globals we have also seen and we have
looked at how to use it and assigned to

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global variables of course in our functions built in refers to the name that are built-in to Python

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and that we've been using throughout the course
we've got into the builtins.py module

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quite a few times now to have a look
at the various functions and built-ins are

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the final place that Python will possible look to when trying to resolve a

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name now if you remember and understand that LEGB in order and again that is local enclosing

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global built-ins if remember that
for resolving names and take note of the

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warnings that IntelliJ gives you should be
quite comfortable with the way Python

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scoping works and then should also
be able to use non-local and global variables

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when you need to

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ok finally that brings this section to an end it's been a very very long one we've

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looked at a few modules from the Python standard library and also the

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documentation so that other modules should
be much easier to use when a program

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needs the functionality they provide and
of course we also had a look at GUI

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programming called TKinter we did a lot with that but we are not done with that yet and theirs few more

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things we need to come before you can
use it to its full potential and then

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covered functions showing you how they are or what they are I should say and how to write your own

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and in the process we learn about scope in Python now classes are also

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created a new namespace

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and we are gonna be covering object oriented
programming in classes in the next section

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so I'll see you their