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


On further reflection, the key thing is that an abstract class has one or
more methods that *must* be overridden. So you can explicitly cripple
those methods:

    >>> class B: 
    ...    "Abstract base class: subclasses must define method blah"
    ...    def __init__(self):
    ...       # ...
    ...       pass
    ...    def blah(self):
    ...       raise "B.blah must be overridden"
    ... 
    >>> class A(B):
    ...    def blah(self):
    ...       print 'A.blah'
    ... 
    >>> B().blah()
    Traceback (innermost last):
      File "<stdin>", line 1, in ?
      File "<stdin>", line 7, in blah
    B.blah must be overridden
    >>> A().blah()
    A.blah


Or implicitly crippling abstract methods by not defining them in the
base class:

    >>> class B:
    ...    "Abstract base class: subclasses must define method blah"
    ...     pass
    ... 
    >>> B().blah()
    Traceback (innermost last):
      File "<stdin>", line 1, in ?
    AttributeError: blah


This also works if the base class is abstract with respect to attributes:

    >>> class B:
    ...    "subclasses must define attribute KNOWN_KEYS"
    ...    def __init__(self):
    ...       self.keys = {}
    ...       for k in self.KNOWN_KEYS: self.keys[k] = None
    ... 
    >>> B()
    Traceback (innermost last):
      File "<stdin>", line 1, in ?
      File "<stdin>", line 5, in __init__
    AttributeError: KNOWN_KEYS
    >>> class A(B):
    ...    KNOWN_KEYS = ('one', 'two', 'three')
    ... 
    >>> A()
    <__main__.A instance at 8070e20>
    >>> 


Personally, I would not bother enforcing abstractness explicitly.
The base class documentation should say "abstract"; either people follow
the documentation in order to make a correct subclass, or their code breaks
and they then read the documentation!

-Steve

-- 
Steve Purcell, Pythangelist
Get testing at http://pyunit.sourceforge.net/
Any opinions expressed herein are my own and not necessarily those of Yahoo