[Python-3000] Generic function PEP won't make it in time

Christian Heimes lists at cheimes.de
Fri Apr 27 19:28:00 CEST 2007

Phillip J. Eby wrote:
> As someone with more recent background in Java than C++, I find the idea of 
> abstract methods having an executable implementation to be quite confusing, 
> and suspect that other people with that Java or C# background will do the 
> same thing.  That is, skim the explanation and miss the significant 
> difference between the C++ way and what they're used to.

 From the naive point of view from somebody with less background 
knowledge about ABCs, abstract methods and generics I expect that

* an abstract class can't be instantiated. An abstract class must
   be sub-classed before it can be instantiated.

* an abstract method has no implementation and simply raises a
   NonImplementedError based exception.

* a class with abstract methods is automatically an abstract class
   unless generics are entering the game of class creation. When a
   generic overwrites an abstract method the class is a concrete
   class but when the abstract method is called an exception is

class AbstractEgg(abstract=True):
     """Abstract class, cannot be instantiated
     def egg(self, foo):


class AbstractEgg:

class Egg(AbstractEgg):
     """Subclass can be instantiated

class AbstractSpam:
     """Abstract class because it contains an abstract method
     def egg(self, foo):

class Spam(AbstractSpam):
     """Still an abstract method because it doesn't overwrite egg()

class GenericSpam(AbstractSpam):
     """Subclass can be instantiated

     I'm not sure about the @generic syntax but it's just an example
     def egg(self, foo):

 >>> gs = GenericSpam()

This works:
 >>> gs.egg(1)

This raises an AbstractMethodException(NotImplementedException)
 >>> gs.egg('foo')

Just my 5 cents from an ordinary Python user who is trying to learn ABCs 
and generics.


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