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:

 >>>​​ ​​str.capitalize(​​'browning'​​)
 >>>​​ ​​'browning'​​.capitalize()

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:

 >>>​​ ​​Book.num_authors(ruby_book)
 >>>​​ ​​ruby_book.num_authors()

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

 class​ Book:
 """Information about a book."""
 def​ num_authors(self):
 """ (Book) -> ...

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