Author Dusty Phillips Reviewers

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All this line does is get the set of methods and properties that the class has, including 
any parent classes in its class hierarchy:
if set(cls.__abstractmethods__) <= attrs:
This line uses set notation to see whether the set of abstract methods in this class 
have been supplied in the candidate class. Note that it doesn't check to see whether 
the methods have been implemented, just if they are there. Thus, it's possible for a 
class to be a subclass and yet still be an abstract class itself.
return True
If all the abstract methods have been supplied, then the candidate class is a subclass 
of this class and we return 
True
. The method can legally return one of the three 
values: 
True
, 
False
, or 
NotImplemented
. 
True
and 
False
indicate that the class is
or is not definitively a subclass of this class:
return NotImplemented
If any of the conditionals have not been met (that is, the class is not 
MediaLoader
or not all abstract methods have been supplied), then return 
NotImplemented
. This 
tells the Python machinery to use the default mechanism (does the candidate class 
explicitly extend this class?) for subclass detection.
In short, we can now define the 
Ogg
class as a subclass of the 
MediaLoader
class 
without actually extending the 
MediaLoader
class:
>>> class Ogg():
... ext = '.ogg'
... def play(self):
... print("this will play an ogg file")
...
>>> issubclass(Ogg, MediaLoader)
True
>>> isinstance(Ogg(), MediaLoader)
True
Case study
Let's try to tie everything we've learned together with a larger example. We'll be 
designing a simple real estate application that allows an agent to manage properties 
available for purchase or rent. There will be two types of properties: apartments and 
houses. The agent needs to be able to enter a few relevant details about new properties, 
list all currently available properties, and mark a property as sold or rented. For 
brevity, we won't worry about editing property details or reactivating a property
after it is sold.
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Chapter 3