Abstract classes?

[Skip Montanaro]

    Daniel> class Observer:
    Daniel>    def update( self ):
    Daniel>       raise "sub-classes must over-ride this method"

You might want to raise the standard NotImplementedError.  From its doc
string:

    >>> help(NotImplementedError)
    Method or function hasn't been implemented yet.

Skip Montanaro | http://www.mojam.com/
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