[issue3263] Odd code fragment in ABC definitions

[Raymond Hettinger]

Raymond Hettinger <rhettinger at users.sourceforge.net> added the comment:

I don't think we should build in explicit support for bad designs that 
violate the Liskov substitution principle. Are there any valid use 
cases for wanting non-hashable subclasses of hashable classes?

If for some reason, this feature survives, it would be better to use 
NotImplemented instead of None.

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versions: +Python 2.6 -Python 2.5

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