Writing a Method in Class Book

As you saw in Chapter 7, ​Using Methods​, there are two ways to call a method. One way is to access the method through the class, and the other is to use object-oriented syntax. These two calls are equivalent:

We’d like to be able to write similar code involving class Book. For example, we might want to be able to ask how many authors a Book has:

To get this to work, we’ll define a method called num_authors inside Book. Here it is:

Book method num_authors looks just like a function except that it has a parameter called self, which refers to a Book. Assuming this class is defined in the file book.py, we can import it, create a Book object, and call num_authors in two different ways:

Let’s take a close look at the first call on method num_authors:

The book part says to look in the imported module. In that module is class Book. Inside Book is method num_authors. The argument to the call, ruby_book, is passed to parameter self.

Python treats the second call on num_authors exactly as it did the first; the first call is equivalent to this one:

The second version is much more common because it lists the object first; we think of that version as asking the book how many authors it has. Thinking of method calls this way can really help develop an object-oriented mentality.

In the ruby_book example, we assigned the title and list of authors after the Book object was created. That approach isn’t scalable; we don’t want to have to type those extra assignment statements every time we create a Book. Instead, we’ll write a method that does this for us as we create the Book. This is a special method and is called __init__. We’ll also include the publisher, ISBN, and price as parameters of __init__:

Notice that we can include doctests for methods just as we do for functions. Notice also that we do not specify the type of the first parameter of a method, since its type is always the class in which it is defined.

This module contains a single (complicated) statement: the class definition. When Python executes this module, it creates a class object and assigns it to variable Book:

Method __init__ is called whenever a Book object is created. Its purpose is to initialize the new object; this method is sometimes called a constructor. Here are the steps that Python follows when creating an object:

1. It creates an object at a particular memory address.
2. It calls method __init__, passing in the new object into the parameter self.
3. It produces that object’s memory address.

Let’s try it out in the shell:

The following image shows the memory model that results from this code:

Let’s trace method call python_book.num_authors(). (As a reminder, this is equivalent to Book.num_authors(python_book).) Python first finds the object that python_book refers to and calls its method num_authors. There are no explicit arguments, so Python only passes in the Book object that python_book refers to, assigning that object to the self parameter:

The return statement, return len(self.authors), is then executed. The expression, len(self.authors), is a function call. Python evaluates the argument, self.authors, by finding the object that self refers to and then, in that object, finds instance variable authors. This is a list, and the length of that list is the value that Python returns, as shown here:

With constructors, methods, and instance variables in hand, we can now create classes that look and work like those that come with Python itself.