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"implemented by subclass"

Erik Max Francis max at alcyone.com
Sat Feb 16 18:35:08 EST 2002 
phil hunt wrote:

> Consider the following python code:
> 
> class Log:
>    def write(self, text):
>       """(implemented by subclass)"""
> 
> Is this the most pythonic way to do it? If not, what would be
> better? How about rasing a ShouldBeImplementedBySublass exception?
> 
> How do other people do it?

I use raise NotImplementedError, and also typically put a test in the
constructor:

	class Log:
	    def __init__(self):
	        if self.__class__ is Log:
	            raise NotImplementedError
	        ...

	    def write(self, text):
	        raise NotImplementedError

The one drawback I know of here is that this can have a somewhat
(naively) unexpected effect if you're using mutiple inheritance; in the
case where you are deriving from to classes, one of which implements a
method and the other does not (with the raise NotImplementedError stub
above), it's best to have the new class (derived from both) explicitly
invoke the right method in its parent class, otherwise the lookup
mechanism may find the first method which has a definition, but which is
in fact the stubbed-out NotImplementedError method as above!

-- 
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