idioms for abstract base classes

[Steve Purcell]

Robin Becker wrote:
> what's the best way to spell an abstract base class so that
> 0) base is B and A inherits from B
> 
> 1) doing B() causes an exception
> 
> 2) most of the initialisation code is common in B


It's not often worth trying to enforce abstractness, but the following
might work for you:


    >>> class B:
    ...   def __init__(self, *args, **kw):
    ...      if self.__class__ is B: raise "B is abstract"
    ...      self.args = args
    ... 
    >>> B()
    Traceback (innermost last):
      File "<stdin>", line 1, in ?
      File "<stdin>", line 3, in __init__
    B is abstract
    >>> class A(B):
    ...    pass
    ... 
    >>> a = A(1,2,3)
    >>> a.args
    (1, 2, 3)
    >>> 


-Steve

-- 
Steve Purcell, Pythangelist
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