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so I mentioned at the end of the previous
video that there was a useful

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application where recursive functions can come
in quite handy and that is dealing

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with files or specifically getting a
directory listing of a computer's file

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system now if you think about it modern
file systems are recursive in the sense

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that a directory can itself contained
other directories so once the

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functions listed all the objects in in
one directory you can call itself again

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for all the objects that are themselves
directories so you can see that's a

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really good use for a recursive function so what we are gonna do is
going to use the OS module to get the

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directory listing that's also useful for
checking if something is a directory so

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the function that we gonna write will
also have verified that the first directory

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passed to it does in fact exist before
attempts to do anything else it now before I

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go any further theirs another
example of recursion and that also

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provides an neat way to use a function
that's nested within another function if

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you really wanted to write a program to
visit all subdirectories then the OS

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module provides a walk function that
will do that for you and in fact I'll

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start off with that with the code we are about to write and I just want to bring up the documentation on the screen

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because like always its a good habit to
get into to check out documentation so

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bring it up on the screen paste it in my
browser you can see the link there we

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go so you can see some pretty useful information about the OS

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library that comes as part and parcel of Python

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and if we scroll down we should see the os.walk generate the file

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names in the directory tree by walking a
tree in a top-down or bottom-up so

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that's why we are gonna look at so again
when in doubt refer to the documentation

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its good way of learning more
information about any libraries that your

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about to use ok so let's go back and I'm gonna
create a new module and I'm going to I said module but I'm

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gonna create a new Python file I mean so....

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I'm gonna call this one filesanddirectories but it doesn't matter what you call it.....and lets start doing some coding so

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so firstly we are gonna import the os because we need to use that library...

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we're going to start with the os.walk so.....

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....

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.....

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....

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....so there's our
basic applications so the os.walk

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function as you can probably gather now
returns a list of tuples each tuple

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contains a directory name and also 2 lists
the first list the name

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is directories and you can see that
on line 6 because I'm referencing that and

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the second one its pretty obvious
on line 8 that we're dealing with the list of

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files was so there are the 2 list that also included in

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the tuple now if you have to work with the
file system the os module is worth reading up

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on and the

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reason is it contains a lot of useful
functions including this os.walk one

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so to see what walk is actually
doing we are going to create our own function

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and actually recurse through all
subdirectories and print out the contents

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but before we do that lets just run it
as it is now to see what it actually

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does so gonna do that.....and you can see on the right hand side the

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output has work so it's obviously
defaulted to the current folder and you

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can see we got a list of files and theirs
actually not really much in the way of

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directories their just to confirm that we
go back

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and have a look in here we click in reveal in finder and go into scope and namespaces and obviously this was the

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directory that it was dealing and hasn't really found much hidden files their starting with a dot on a MAC

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are hidden which is why we can't see those so let's continue on now

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and what we'll do is we'll create a
function now and put a bit more

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coding into it and will make a look
it a bit prettier by indenting the list as

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we enter a director etc so lets look at doing that

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so gonna come up here and will define a function so...

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....

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.....

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....

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.....we are going to write that shortly

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so in this particular function we are doing a
small amount of initialization the

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important bit is obviously making sure
that the starting directory that's

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passed to this function actually does
exist once that's done its gonna call a nested

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function that will recursively visit all
subdirectories and display its contents

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now the thing is the DIR_list function that we are about to

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call on line 8 is only actually
needed by this function so that's a good

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reason for defining it within list
directories so basically a function

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within a function so here is how we go about
doing that

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so we actually start in the list _directories function and we type.....

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.....

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...bearing in mind we already validated that the directory

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exists and we will by the time we call dir_list and will do a check their so put...

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....

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.....

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....so you can probably gather that one is

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doing that's actually putting together
the directory and the file and

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making a path for it so we are doing a
test here then to say whatever that was

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actually found is that directory
entry because an entry in a particular folder can be

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another folder or file so we are doing a check first

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to see whether it is a directory I'm
using the term folder but obviously directory

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and folder are terms interchangeable
terms they both mean the same thing so we are

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determine that it directory so....

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....

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...a hint with a tab its going to make
things look prettier and be able to indent correctly and

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in actual fact the next line......and

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we are getting an error their and we'll talk
about this shortly so...

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so using recursion we are calling back and using the
same function again and then we put....

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.....

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....

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....so that
should be our function so basically

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each time the inner function in a
function rather finds a directory

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it increases the tab stopped by 1 and calls itself to visit the new directory now when

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it returns it decreases the tab stops so you can see here on line 11 we're at

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increases the tab stop it calls the
function calls itself again and it

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decreases the tab again so it turns out to
be much simpler solution than trying to

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keep track of which directories which
already have been visited and shows

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how recursive functions can be very useful when
dealing with recursive structures such

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as the file system now of course that's all
it work of course so it's a good idea

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but IntelliJ as you can see on the screen
is actually showing errors and obviously

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the code isn't going to work at
the moment

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know if you're familiar with other
languages that allowed nested functions

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2 such languages are Algol and Pascal

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then you maybe a bit confused by the error so it's
saying that it can't resolve the

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references to tap stop but tab stop is clearly
defined in the outer scope as we can see

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on line 18 and in fact it looks
very similar to the situation we had

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with local variables shadowing those
with the same name in the outer scope

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and that's why IntelliJ was careful to refer them as variables in

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the outer scope rather than the global
variables so this is the same situation

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but the outer scope here is the function
list_directories of course that

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is defined on line 3 just need to make another line their to get rid of that error and the other thing we are gonna do down here now is

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to delete this code we don't need that anymore and we are going to be calling this when we figure

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out what's going on here with list_directories with the current folder

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so basically the errors that has coming up is
unresolved reference tab_stop as you can

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see their and that's what's actually causing the
problem now if we define a tab_stop on line

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within on line within the actual
function the inner function we can see what

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happens so lets try doing that so....when we do that we

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get the error that we've seen before shadows name
tab_stop from out of scope and the

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reason for that is now we've done that Python's created
a local variable called tab_stop

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just like it did when we
tried to assign it to a global variable in

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the blackjack program and as a result
its warning us that it's shadowing the

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variable in the outer scope which of course
is the variable on line 20 which is in the

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list_directors function so at this
point you might be wondering well maybe

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I need to use a global declaration on
line 7 to fix this lets try doing that

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....and you notice we got a different error here and will get rid of the equal 0...

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and if we try and run this now we actually get an error

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you can see the named error is named
tab_stop is not defined

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so clearly there's still something
going on here and the warning here is

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that remove redundant parenthesis and
so I'll just fix that up and that shouldn't

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have an extra bracket their so I am to use to
programming on other languages so lets fix that up

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at this point even though we know its
crash because I have shown you that if we

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run this again just to confirm its still crashing we are getting this variable error tab_stop is not

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defined and obviously there's now no errors
in the program but clearly theirs something

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still going on that is causing this
error so with Python 2 we would have been

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pretty much stuck here now their are ways to
get around this and of course we could

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just create a global variable but
Python 3 introduces another way to

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specify the scope of a variable in situations
like this

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and what we need to do is change the
global reference on line 7 to non-local

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if we do that we should find it works we type....and now if we run it

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you will find its now working correctly you can see the
directory listings working we are getting

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embedded file names and everything seems
to be working better than it did before

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so lets end the video here in the next video we're going to continue on we're

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going to just sort of talked about why this is
working and understanding the scope and

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how this non local keyword is helping
this to work so see you in the next

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video