Author Dusty Phillips Reviewers

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pass
@classmethod
def __subclasshook__(cls, C):
if cls is MediaLoader:
attrs = set(dir(C))
if set(cls.__abstractmethods__) <= attrs:
return True
return NotImplemented
This is a complicated example that includes several Python features that won't be 
explained until later in this book. It is included here for completeness, but you don't 
need to understand all of it to get the gist of how to create your own ABC.
The first weird thing is the 
metaclass
keyword argument that is passed into the 
class where you would normally see the list of parent classes. This is a rarely used 
construct from the mystic art of metaclass programming. We won't be covering 
metaclasses in this book, so all you need to know is that by assigning the 
ABCMeta
metaclass, you are giving your class superpower (or at least superclass) abilities.
Next, we see the 
@abc.abstractmethod
and 
@abc.abstractproperty
constructs. 
These are Python decorators. We'll discuss those in 
Chapter 5
, 
When to Use
Object-oriented Programming
. For now, just know that by marking a method or 
property as being abstract, you are stating that any subclass of this class must 
implement that method or supply that property in order to be considered a
proper member of the class.
See what happens if you implement subclasses that do or don't supply
those properties:
>>> class Wav(MediaLoader):
... pass
...
>>> x = Wav()
Traceback (most recent call last):
File "", line 1, in
TypeError: Can't instantiate abstract class Wav with abstract methods 
ext, play
>>> class Ogg(MediaLoader):
... ext = '.ogg'
... def play(self):
... pass
...
>>> o = Ogg()
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Chapter 3