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in the previous video we have a look at

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polymorphism and we saw how different

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objects can inherit similar behavior

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from a common base class so although ints

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floats and lists are different types

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they can all be printed

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that's because they will provide a

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string or STR method if a type doesn't

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provide a specific implementation of str

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string method it ends up

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getting inherited from the base object

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class so python isn't really interested

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in what type something it focus

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instead on how things behave so again if

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it quacks like a duck it's a duck when

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we finish the last video by giving our

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penguin class the same methods as the

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duck class so that penguin would behave

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the same as a duck so inheritance isn't

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the only way to implement polymorphism a

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class can behave in the same way as

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another class as long as it's got the

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necessary methods and data attributes so

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relating this back to our enemy objects

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we can say that a troll is an enemy and a

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vampire is an enemy so inheritance

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results in a is a relationship and

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similarly a penguin is a duck at least

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as far as that test_duck

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functions concerned so in this video

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we're going to start having a look at

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two other object orientated terms

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composition and aggregation now

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composition and aggregation are used when

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you have a has a relationship rather

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than a is a relationship so if we go

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have a bird's wings we wouldn't say that

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a duck is a wing would say that duck has

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a wing now if you're unsure about

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whether you should use inheritance or

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not then those two ways of looking at

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the relationship will normally make it

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obvious whether inheritance is

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appropriate or not what I'm going to do is

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going back over here now to our duck

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.py again I'm just going to comment

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at this code at the bottom for new so

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what I'll do is I'll comment the

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test duck altogether and we'll just

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like Donald in will just comment out

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percy for now and I'll just delete the call

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to test _ ducks so we are just left

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now with defining donal alright so

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let's add a wing class to the start of a

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main file and we are going to give duck

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class wings so at the top

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here.....

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...

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...

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....

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...

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...

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ok the couple of extra lines to keep intellij
...

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happy now this is simplified

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slightly so whether wing is capable of

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lifting an object such as a duck depends

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on things like the weight of the object

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the surface area and shape of the wing

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and its speed through the air and we

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could apply Newton's law and put in the

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correct formula to decide if our wings

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capable of flying when it's attached to

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various birds but I've taken a much

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simple approach for this example if the

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ratio of the wings thrust to the weight

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of the bird is greater than one we're

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going to say that the wing can fly if

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it's equal to 1

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the bird has to flap hard to get off the

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ground and if it's less than 1 then the

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wing won't lift the bird

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ok now we get to see composition in

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action I'm going to go through all the

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code once we've seen it working since it

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makes more sense once you see what does

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so go back to the duck method and need to

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add a constructor....

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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 let's just run this we get the

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result we this is fun so and again we

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this is fun is coming from self .

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ratio being greater than one there so what

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if we actually done here

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well in the init method for duck class

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on line 17 we create a new

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wing object and assign it to the _

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wing attribute of the duck class now

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any duck objects we create will have

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their own wing object and can use the

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attributes of a wing including this case

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the fly method now when a class

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contains another object like this

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that's called composition now we could

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add a leg class and a beak and build a

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complete duck out of component parts

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the duck would then be composed of

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other classes and would have few or no

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attributes of its own that weren't provided

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by the composing classes now that's

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probably a bit extreme for a duck but

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could make perfect sense if we were for

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example representing an HTML document so

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let's actually do that instead seeing

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composition being used in a more

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realistic situation is a good way to get

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a feel for how powerful a technique can

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be so let's create a new python file

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new python and will call this one

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....now I'm not going

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to actually explain html here

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most people already know the basics of

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HTML even if you haven't created any web

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pages using it a HTML document is

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basically a structured document that

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uses tags to enclose text the browser

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uses those tags to work on how to display

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the text let's just have a quick look at

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the structure of HTML document open a

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browser and do a search here for HTML

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document structure the first results

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telling us what it is without us having

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to follow the link and probably the

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better one here definition is

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this one here the global structure of

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HTML document i could click on that

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and look and these three lines here

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probably sum it up pretty well there's

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three parts that a HTML document must

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have a line containing HTML version

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information that's called the document

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type definition or DTD a header section and

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a document body and we've got an example

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on the screen here so at this point what we

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can do is we can say that a html

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document has a DTD and it has a header

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and it has a body when you've got has a

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relationships like this that's an

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indication that we could consider we

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should consider using composition when

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creating a class

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ok so the basic building block of HTML

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is the tag

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let's go back to code and start creating

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a tag

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class.....

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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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....

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we also want to create the string....

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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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....

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...

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...

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ok now if you're not familiar with HTML

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tags they have a name which is enclosed

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in angle brackets the tag is close by

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using a similar tag but with a slash

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before the name that's what we're trying

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to do the definitions on line 4 and 5

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if you go back to our browser again

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in the structure here and you can see

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the definition their now the DTD looks

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quite complicated but it always takes

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one of the few forms so when

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getting a webpage you just copy and

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paste it the rest of the documents in

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close as you can see there in HTML

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starting tag up here and ends with the

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closing HTML tag down here notice how its

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got a / in front of it for the closing

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one and obviously we can see that on how it

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works in the example here now the tag

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names aren't case sensitive so i'm going to

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be using lower case for no other reason

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that I prefer typing it in lowercase now

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within the HTML tag their is a head tag as you

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can see here followed by body tag down

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here the contents of the head tag up

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here include a title and the bodies

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contents here a single paragraph would

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start with that p with the less than

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greater than sign so the P tag in other

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words strictly speaking looking at this

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code there should be a closing P

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tags in other words a less than sign / p

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greater than sign closing tag but the

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HTML specification isn't very

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restrictive so you'll often find that

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tags aren't closed xml on the other hand is

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stricter and all tags there must be

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closed so as this example is at the

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moment you couldn't use an xml parser to

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read this html code it will

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actually fail because there's no

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closing P tag here

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alright so that's the sort of thing our

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classes are looking to create or going

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to create let's go back to intelliJ

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now the tag class constructor

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initializes the init method up here

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on line three accept a name for the tag

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and its contents it then uses string

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formatting to create the start and end tags

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and stores the contents in the self dot

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contents data attribute now the string

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method the STR method will return the

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complete string for the tag so that's

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the opening tag followed by the contents

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and finishing with the closing tag and

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as i mentioned i'm also giving the

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class a display method just prints the

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instance out as we saw earlier the print

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function we use the result of the string

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method to do that

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alright so next we need a doc type class

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so let's go ahead and create that it's

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gonna be class....

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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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alright so doc type inherits from tagged and

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uses the super method to initialize

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itself with the DTD that we saw on the

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webpage now doctype doesn't have any

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contents and there's also no closing tag

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so we are setting that the closing tag

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the end tag on line 19 to an empty string

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but hang on though I've just extended

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the tag class doesn't that mean we're

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using inheritance here rather than

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composition

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yes we are now the two approaches are

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exclusive and you can create a class

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hierarchy using inheritance and then use

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those classes in a composition

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relationship to build up another class

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when using composition you have to have

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some classes to compose a class out of

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so our HTML document class will be

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composed of a DTD the header and a body

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at the moment we're creating those

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classes

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we're going to come to composition when

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we create the HTML . class in a minute

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for now though let's just finish off

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this video by creating the head and body

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classes and turns out the head is going

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to be very simple class to create we're

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going to go ahead and do that it'll be....

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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 just going to be there

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the head opening and closing tags with
....so just going to be there

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no content that's what we've actually

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done there in the head class if we want

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this to be complete we should actually

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add similar code to what I'm going to

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write in the body class but I don't want

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to get distracted by those sorts of

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things that go into the header so to

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keep things simple our header is going to be

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empty for now although I can sense the

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a challenge is coming up

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ok let's finish off now and create

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the body class and end

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the video move on to the next

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one so class body tag....

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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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....

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alright so let's add method to add the

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tag we're going to put....

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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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...

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...

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alright let's just discuss this briefly

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before we end the video so once again for

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the body class were inheriting from the

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tag class but we initialize the

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contents to an empty string when calling

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the super method because we don't what

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the contents to be created just yet and

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that's because we're going to store all

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the contents and list as you can probably

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ascertain in the attribute self dot

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_body_ contents so we

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initialize that to an empty list in the

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constructor can see them doing that on line

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32 now the add _ tag methods

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used to add the content to our body and

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we give it a name of a tag and the

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contents and it creates a new tag

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instance and appends that to the body

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contents list i'm going to move swiftly

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on here but you may already spotted this

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is an example of composition absolutely

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it is and I will come back and talk

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about that more in the next video i want

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to show you the use of composition in

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the document first because that's a much

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clearer example so moving on in the

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display method as you can see there on

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line 38 overrides the method from the

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tag superclass so it builds up the

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contents attribute by iterating over our

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body contents list and adding each tag

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to the contents and once thats done it

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then uses the superclass display method

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to display the body tag with the full

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contents

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now if you were going to fully implement

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the header then we could give our head

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class the same ability to add tags and

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build up its contents in fact we might

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create a special kind of tag class that

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can contain other tags and have both

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head and body inherit from that but we

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are trying to keep things simple here

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alright so I'm going to finish the video

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here now in the next video

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now that we've got all the classes that

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we need to make up our HTML document we

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can create the actual document class so

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let's work on that in the next video