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So, welcome back.

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Now what I want to do is I want to dig into a little bit

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about how you as a developer of a Django application

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interact with Django itself with objects.

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So, just to recall,

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if you're looking for the downloads, dj4e.com is the place to go.

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We are working through the Django tutorial

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and I'm pretty much no further than part 2 in this particular one

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because it really not worried about building the Django app,

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I'm really interested in how objects work.

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So, one of the things that you can do because Django is really cool open-source,

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is we're actually going to look at the source code to Django.

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Django is open-source and has 26,000 different separate modifications,

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almost 1,700 contributors, and it's a lot of source code.

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It's probably a million lines of code.

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And so this is real enterprise software,

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it's very exciting stuff and open source

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is really cool because we can look inside Django.

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Django is not a magical thing that we can't see

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and we can learn from it and understand it.

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So, what I'm going to try to do,

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is I'm going to take you inside Django

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and our application that we write is going to be really simple.

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It's just pulls application that just prints out,

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"Hello World" because this isn't a Django class,

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this is really how object-oriented works inside Django.

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So, what we're doing here is if you

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read the Django documentation and you read the Django tutorial,

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what you find is that,

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oh okay there's this big thing called Django and you got to write these files,

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this file URLs that routes things and models that creates

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database tables and allows you to read and write data from those database tables,

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and the views that shows what the user interface of this is.

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And so, you're supposed to create these files.

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So, I'm going to focus mostly on the models file and talk

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a little bit about what's going on inside of Django and how Django

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in effect creates classes and then you create classes that derive from

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the Django classes and then you can create

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objects that are based on your classes and then use those objects.

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We're going to dig in to the source code to Django.

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So, this particular one,

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let's take a look at this base.py,

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let me show you the models file that's our code.

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So, here's our models file and if you read the documentation and in Django it says,

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well if you want to create a database table,

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it's called a model and the model view controller.

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We're going to make a database table, has question_text,

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publication date, and then have some methods inside of it,

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like how to print this particular object out as a string

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and then this was published recently

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is something that they're going to use in there.

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Then we're going to another table called choice

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and it's going to have a question column and a text column.

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We can see this, there's this thing called migrations that reads this and creates tables.

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And so, in here,

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we have polls2_choice_question,

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polls2_choice which has those columns and it's created this database table for us.

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This thing not only allows us during runtime to read and write from the database,

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but it also at setup time it creates things for us.

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And so these two database tables that I've got sitting in my database here were created

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from this object because it runs one process to read

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through your objects then create database tables.

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It's another process while it's doing request-response cycles,

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then it actually reads and writes the data in the database.

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So that's one nice thing and so models.Model is a class.

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If you just look at this, we're creating a new class,

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that's Question, it's our class

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and we're extending the models.Model class

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and we're doing this to inherit lots and lots and lots of functionality.

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And then, the definition of this is just variables models.Charfield,

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that's a variable or constant constant with

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some stuff and these are objects and all this stuff is sitting there for us.

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It's almost like a dialect of Python

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that is specifically to define these database models.

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So, if you know that this is a class and that it's extending another class,

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let's go find the source code to that class.

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So, now what we're going to do is we're going to go into the Python GitHub repository

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and here on line 383 in the base.py,

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this big long thing if you're looking for it and its linked

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here from this slide right here and now this is Django code.

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This is the code that was written by the Django project.

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And so, they made this model and if you look at this,

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you've made simple classes.

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This is not that different than the classes you made.

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So, you have _init_ method and the first parameter is self.

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Now that should look familiar to you and then you have a couple of arguments,

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and then we got a whole bunch of code.

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And so, if we start looking down here,

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this is just the constructor.

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So, part of the goal of object-oriented programming is you get to write

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a little bit of code that leverages

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hundreds and hundreds of lines of code that somebody else,

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some other really sharp people,

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let's see if we can find the contributors.

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So, this is a list of all the people that built this.

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These people know a lot,

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there's a whole bunch of people and we can

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see their work and what they're doing and how long they've been

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working and they're doing things like reading all these documents,

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like the hypertext transfer protocol

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and this and that and knowing how to talk to databases,

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and how to talk to SQLite database and how to

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talk to MySQL database and Postgres database and Oracle database.

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You literally don't need to know anything.

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And so what they're doing, is they're building

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a really large object or

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class that then you can extend and you don't have to write all this code.

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This is very general-purpose code.

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Part of the goal of writing an object is to hide this from you.

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Now, we can look at it,

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you shouldn't have to look at it.

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You should be able to look at

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the documentation i.e the documentation on these little tutorials.

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I'm just trying to give you the sense that,

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here we go and so this is still the constructor.

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This is the initialization that constructor,

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so now we now have a method and so these are

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other methods and these are things that you'll recognize,

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the double underscore string underscore that's what happens when we

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convert a model object to a string underscore underscore.

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So, this is no different than if you convert a dictionary to a string.

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There's a little method inside of

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Python that's double underscore string double underscore.

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eq, that's comparing equality.

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Hashing has to do with how dictionaries work, getstate,

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setstate and all these things are just the things that if I am the Django developer,

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I have to do this because that lets me mount my class into Django the

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right way and these are methods that would be documented like serializable value-

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Save, and there's another like whole bunch of stuff.

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See, how much work it save base.

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If you're probably looking at the Django documentation to see all this stuff.

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Save parents, were still in the model object.

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Right? Next to prepare database save, clean, validate,

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unique, and so this is just code that was written by these people.

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Where are they? All right.

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All these people wrote all this code,

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and read all the documentation and tested the heck out of it.

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You just write this code right here.

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You extend, and you inherit all that stuff.

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That does things like creates other tables which I showed you.

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It actually hooks into an administrator interface.

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Let me see if I can show you that.

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This is an administration interface.

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So, this user interface is part of your Django application that came from Django,

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that's the admin tool,

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the admin panel within Django,

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and your model is now in there.

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All right?

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So, you got some question text and date published et cetera.

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So, all you did was created one little file.

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Right? Django is this magical thing,

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administrator interface, database creation,

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table creation, et cetera.

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You created this little file, and away you go.

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Okay? So, then I'll show you one more thing.

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Just the pattern you'll see it's kind of the same.

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Is in this views.py file.

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So, this is returning a response when you go here into the polls to application.

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Right? It returns this response.

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But if you were to look at this in the developer console under network,

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you would see that there is a whole bunch of stuff that you going to do to do HTTP.

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We sort of saw that in the first video.

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Like the response headers,

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all these things what the content is,

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what the type is, the date

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these things and there's many other things that might be necessary.

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So, you could learn how to write all these things,

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or you could basically say, you know what?

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I'm just going to inherit the obvious things,

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and I'm going to read the documentation on HttpResponse.

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So, this is an object,

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this is the constructor to that object,

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and we're going to send in a string to the constructor.

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So, with HttpResponse we could create it,

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we could set headers,

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we can do all kinds of things in it.

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Not just the text, but all kinds of other things that have to do with the HTTP protocol.

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So, somewhere inside of Django,

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they have defined this object that's the HttpResponse.

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So, I can show us that one as well.

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Take a look at the HttpResponse.

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We're using it in a very simple way.

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But, if you look inside Django,

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you see there's a lot to it.

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So, here we go is content,

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is a set of bytes in this case,

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and that's the constructor,

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then there's serialization et cetera.

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So, all those things are part of that object and HttpResponseBase,

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and we'd go find that one which I think is just up here.

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So, even within this file they're using one class HttpResponseBase,

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content-type, that's the thing about its text/HTML_status,

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that was the 200 okay et cetera charset UTF8.

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So, we're setting that thing up that's a constructor here.

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If you were looking at the documentation, you will see that,

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the charset, setting a cookie,

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cookie has to do with the little bit of

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data that goes back and forth between the server and the browser.

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You don't have to know how to do that.

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You can go read that. You go into RFC 2616.

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If I got that,

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look at 2616, and then we could search for cookie.

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How come I don't see that.

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RFC HTTP cookies, which one is it?

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Maybe it's got its own document.

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Yeah, there we go. Rfc 6265.

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So, if you want to set a cookie which is how we log in and out of applications.

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You've to go read this one. How many pages is this?

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It's only 37 pages.

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But it turns out, that thankfully somebody in the Django project, where are we?

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As a little command,

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has a method called set-cookie for us,

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and for you to go look at the documentation.

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If this HttpResponse want to Set-Cookie back in the browser,

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instead of reading all this stuff,

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some kind person who helped write Django wrote all this down,

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and gave you a method called set-cookie.

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So, I'll stop there,

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I'm not really trying to get you to sort of be a wizard in any of this stuff,

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I'm trying to give you a little tiny places where you can jump in and learn more.

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The basic idea is there's lots of complexity to this, lots of documentation,

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lots of rules about how to comply to this stub,

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and Django has made it,

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so that's all hidden behind this abstraction boundary.

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You have objects, you make those objects.

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So, we played with the models object,

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and then we played with the HttpResponse object,

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and we can use as much or as little,

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we didn't have to use the cookie and the response object.

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We can use as much or as little of that stuff as we want.

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That's the beauty, is that all that complicated detail is hidden inside

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Django with the convenience of we can use as little or as much of it as we want.

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So, I hope you didn't try to like write Django because of this.

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I just wanted to make a little connection with how you work with large frameworks,

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or large libraries, and how object-oriented programming,

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and the patterns of abstraction, and hiding,

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and inheritance can be put to good use both by building libraries,

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and then by using those libraries.

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I hope you found it useful. Cheers.