WEBVTT

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so as I mentioned the album classes uses a
list to store the songs and provides a

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method the add song method that we've
create for adding more songs or

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adding songs to the list and it's also got a field for the artist and we

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got an error at the moment and that's because we
haven't created that class so we are going

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to be doing that in this video now having an artists object
stored in the album object can cause

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problems because the artist class will also hold the
list of the artists publish albums but

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I'm gonna talk more about that when we've seen
the artist class now one thing worth

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mentioning that before we get to that is
that the error in the doc string and that is up

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here you can see that if we flag is it says unresolved reference album_name so this

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really highlights the fact that the doc strings
aren't just ordinary strings but they

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really do have a special significance
looking at the init method as you can

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see here on line 30 or 30 downwards we can see that the data attributes of the

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class a name year artist and tracks but I
incorrectly referred to the album_name

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attribute in the doc string so IntelliJ has flag a warning unresolved reference album_

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name so it's unusual for the
content of strings to be check for

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syntax errors but doc strings are used by
a number of tools including IntelliJ

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itself now IntelliJ uses the doc strings
of the built-in modules to produce the

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intellisense tooltips that appear when
you're using Python built-in

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functions and classes but as the
built-in themselves are actually written in

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C IntelliJ can't examine the real source built-ins.py is a stub provider so that you

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can checked that various declarations and we have
talked about that before now its easy

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discussed this with built-in with
built-ins modules on the screens so I add

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a call to the code down here on line
54 to do that so we can see what we are talking

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about so type...

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again we get that same warning here unexpected
argument so we can command or

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control-click on album you get access to
the documentation that way but also

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going back and closed it down or come
down here again we click on Help

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commander or control click on Help we can
see the definitions in the built-in.py

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you can see that there's a
comment field next to it so that the

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real signatures are unknown and has been
restored from the doc string but the

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doc string doesn't list the functions
arguments as a result that's the reason

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why IntelliJ doesn't know that it can
take a parameter and to give the warning

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as such so close it down and go back to song.py and specifically we'll fix this

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error with the album_name
so I'm going to change that to what it should

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have been called with name and we should find that error disappears because of course name

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is an attribute defined in

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our init method you can see the error then goes away
and also delete this help album which

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we no longer need and I what will do
is instead will start typing the new

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class so lets type the artist class

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out so...and I'm going to make a space again so we can see what we are doing here so...

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....

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....

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.....

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....ok we also want to document the method so....

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.....

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.....

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....

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...again this is a good idea
to put that type of documentation in if

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that is the case remember when you do
make the change to go back and revise

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the documentation so all we are doing here is.....

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so theirs not really any checking their....

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so the artist class has got 2 data attributes
the string holding the artist's

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name and a list containing the album objects
and its a standard way to organize your

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record collection now with the albums by
particular group stored together

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compilation albums or the songs by
various artists do complicate things in

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this design attempts to deal with that
by allowing an artist to be specified

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for each song now the major problem with
this design currently is that an artist

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object will have a reference to an album
and that album object will also have a

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reference to the artist now the problems
that that can cause are to do with

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garbage collection so when objects are
no longer used in your Python code they

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still take up memory until they're
ultimately destroyed now many more modern

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programming languages deal with this by a
system known as garbage collection

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and that means that the runtime system
keeps track of variables and reclaims their

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memory when they are no longer being used in the program
anymore and that generally means even

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when they go out of scope or when there is no
longer anything else referring to them

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as an example now if where to create an
album object for say adells 25

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it would have a adell as its artist attribute
however the adellt artist object would have 25

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in its list of albums so when the
program is no longer using these the

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garbage collector might see that
there's still a reference to both

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objects each from the other one and then
ultimately not reclaim the memory

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for either so you can see how would then would
stall both of those entries sort of

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permanently well until your program
finishes and won't claim

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the amount of memory space with that said the
Python 3 garbage collector is quite

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advanced and can actually cope quite well
with these situations but circular

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object references like the one like this that
I just mentioned are still best avoided and

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another problem with circular references
like this is that it can seriously

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complicates saving the objects to file
or to a database so sticking with the

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adell for a moment when we try to write a
adelts artist object then we will go through all the

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attributes and save them into disk when we
get to the album's object the program

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will go through the album attributes and save
them to disk

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in the process it would find an artist
attribute that would then need to be

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saved so it will go through all the
attributes and save them to disk in the

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process finding a collection of albums
each containing a reference to an artist

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object that contains a collection of
objects and so on so Python pickle module

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can often cope with this type
of listing but there are limits and it

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can also get quite confused he can get
quite confused and I think you can too as the

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program in this scenario and in actual
fact I have made it even worse here and

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I've done that by also storing the artist as an attribute of each song in the
album's tracklist so lets just open up

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an image to see this in more detail so
let's just have a look at the class

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structure for what we've come up with
you can see the diagram on the screen so this is a

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little bit complicated and there's
really no good reason for storing the

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artist object in the song class for
example the artists name would be enough

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to allow the actual artist
object to be retrieved

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if more details were required so we are gonna take a
look at that later and just to be clear

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I'm not saying that you should never
allow circular references in your code

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but if you do allow them then it should
be for you know a really good reason with

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that said you should also be aware of
what's going on and also of the potential

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problems that may arise as a result of
using them so we're going to remove the

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circular references a little later but we are gonna leave them in for now so that we can see the program does work as

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its currently set up so we are going to go back to the
code and now that our classes are

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defined let's move on and start doing
something with them know rather than

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doing loads of typing and also because
using classes to pass data is a very good

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way to do it I'm gonna read in the
songs information from a text file now we've got

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a file albums.zip that can be downloaded from the Resources section

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of this video so download that file then
extract the file and then copy that file

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to the same directory as your Python
project

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file in IntelliJ and again you can do
that too easily determine where you are by just

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right clicking the project here

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and coming down to reveal in finder which may be
show files or something like that on a

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different platform click on that and it
takes us into the folder so I'm gonna put the

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actual files in that folder there where my other py files are and  I'm just gonna navigate

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to the file location

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where I've already downloaded the files the album.zip and unzip and copy it in their

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so I'm gonna go to that folder to my downloads
folder

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ok I got albums.zip their so I'm going to unzip that
theirs the albums.txt file that we

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are going to import I'm going to take a copy of that
go back to the path where my project files are and

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pasted that in there and then when I go back to the code in IntelliJ

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we got albums.txt just appeared so
the file if we have a look at it you can see

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it's a tab separate list of songs with each
song records starting on a new line now

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a record here consists of the artists name
the album name the year it was released as

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well as the song title now Python will make  light
work of parsing this kind of data all we

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really have to do is to split each line at the tab character and convert the result to a

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tuple and it's also necessary to
remove the end of line marker from each

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line and fortunately there is again
trivial to do in Python so let's

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create a function to load this data up and
call it only if the program is executed

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as a script so going back to our code again I want to close this text file so here in our

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artists class we are not going to put in
the artist class we are going to start typing it as

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a separate function outside of the class so....

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.....

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....

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.....

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...

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....

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....

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.....

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...so that should give us our 4 fields and as you can see Python makes light work of something that

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would take a lot more code in other
languages.....

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...

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....and that's our method so lets go back

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and confirm this works again we are only running this

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if the programs executed as scripts so we are going to do a test here so...

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....

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...alright lets try running the program

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and we should see that its displays the details for
all the songs from the

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text file so run song and it seems to
have worked ok we've got a list of the

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artists field the album the year as
well as the song title itself so that's

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good that is working well.....so you can

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see there or you could see their that the fields variables seem to be getting the correct values ok so we got

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that working....

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so now we got this working I'm gonna finish the video
here in the next video we're going to

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move on and enhance a little bit more so
if we are gonna start putting the data into the classes

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following naturally from the data so in
other words as each rows are read

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we are gonna create a song object then add it
to the album and when a new album is

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found in the data the current album is going to be store in the artist album list and a new album objects is going to be created

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with that current rows details so lets work on that in the next video