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Constructicons
5:26What if we need something more complex than just a custom init or new method?
So far, all of our methods have
operated on an instance of our classes.
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They're what we might
call instance methods.
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There's a special type of method in Python
though that lets us construct an instance
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of our class without having to
have a class instance already.
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We've seen one of them, new.
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But its specialness
was built into Python.
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We can get this behavior on any method
we want by making what's known as
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a class method.
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Let's check them out in WorkSpaces.
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Class methods give us a slightly
more interesting way to do object
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instantiation.
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Now while they don't replace,
they init in new methods.
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They often act as what's called
a factory for an object.
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For instance let's make a book class.
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Okay, so
I have a file here called books.py.
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You'll wanna go ahead and
make that if you don't have it.
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And I'm gonna make a book, and
I'm gonna make a bookcase, all right.
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So bookcase.
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And let's fill out book real quick.
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So we'll say for the init of a book we
have a title and we have an author.
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And we'll say self.title = title and
self.author = author.
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And let's make a __str__ for this.
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And we'll return that it's blah by blah.
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And we'll format that with self.title and
self.author cool, all right.
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So now let's create our Bookcase.
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So it takes self, and it takes a list of
books, which we're gonna say is none.
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Because you don't want to
provide a mutable object
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as the default argument to a argument.
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So I say self dot
books is equal to books.
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And now let's create our class method.
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So first of all we put this,
and it's @classmethod.
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And I'll explain what that
means in just a minute.
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And then we're gonna do a method here
that we're gonna call create_bookcase().
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And it takes an argument called class,
cls, and
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it takes an argument called booklist.
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books equals an empty list and
for title and
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author in Book list, books.append,
book with the title and the author.
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And then in the end,
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we'll return an instance of
the class with our list of books.
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Now class methods don't take
self as their first argument,
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or the instance, or anything like that.
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Instead they take the class that
they're being called on and
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we can't use the word class here.
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We can't do class because class
is a reserve key word, right?
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Class has to be reserved for this.
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We can't use class here.
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So you'll see cls used a lot.
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It's a very common abbreviation for that.
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You also sometimes see klass,
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that's usually used in more meta
programming kind of things.
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So cls is a good default.
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Inside the method we're not
really doing anything special.
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But notice at the end that we
used the class that was passed in
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to create a new instance.
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And we passed a list of books
that we've now created.
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And that'll use the class's __init__ up here
to do its constructory thing.
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So nice, we've encapsulated or
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contained some logic to make creating
a case full of books easier.
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Now something you might not of seen before
is this @ symbol here before classmethod.
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This symbol marks
classmethod as a decorator.
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A decorator is more or less a function
that takes another function,
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does something with it, and
then usually returns that function.
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The class method decorator, for example,
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does some modifying of the expectations
of Python's object class.
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Which all of our classes
are inherited from, so
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class methods can work how they do.
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Let's try this out.
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So python and
we will say from books import Bookcase.
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And let's make a bookcase and
we won't instantiate the Bookcase.
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We'll just say
Bookcase.create_bookcase() and
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then we pass it our list of books.
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So let's say Moby-Dick by Herman Melville,
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Jungle Book by Rudyard Kipling.
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I have two whole books on my bookcase.
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And now, if I look at Bookcase,
I have this bookcase object.
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And I know that this has
an attribute in there called books.
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So let's say bc.books and
there's two books.
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And let's look at book zero.
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Let's do a string version of that.
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And we get Moby Dick by Herman Melville,
so cool.
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It created the books for us and
put them onto our bookcase.
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There's another special type of method,
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known as a static method, that doesn't
require an instance or a class at all.
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These methods are mostly for
design purposes,
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they let us put code into a class purely
because it belongs with the class.
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Most of the time though,
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static methods can be done more cleanly
as plain old functions and used scripts.
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Static methods can provide a nice
set up design and organization and
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they can be overridden
through class inheritance.
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But most of the time they're
not all that useful.
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