Instances behaviour

[Inyeol Lee]

On Thu, Dec 01, 2005 at 03:51:05PM -0800, Mr.Rech wrote:
[...]
> Suppose I have a bunch of classes that represent slightly (but
> conceptually) different object. The instances of each class must behave
> in very similar manner, so that I've created a common class ancestor
> (let say A) that define a lot of special method (such as __getattr__,
> __setattr__, __len__ and so on), and then I've created all my "real"
> classes inheriting from it:
> 
> >>>class A(object):
> ....     # here define all special and some common methods
> 
> >>> class B(A):
> ....    # this is the first "real" class
> 
> >>> class C(A):
> ....    # and this is the second
> 
> and so on. The problem I'm worried about is that an unaware user may
> create an instance of "A" supposing that it has any real use, while it
> is only a sort of prototype. However, I can't see (from my limited
> point of view) any other way to rearrange things and still get a
> similar behaviour.

>>> class A(object):
>>> ...     def __init__(self, foo):
>>> ...             if self.__class__ is A:
>>> ...                     raise TypeError("A is base class.")   
>>> ...             self.foo = foo
>>> ... 
>>> class B(A):
...     pass
... 
>>> class C(A):
...     def __init__(self, foo, bar):
...             A.__init__(self, foo)
...             self.bar = bar
... 
>>> a = A(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 4, in __init__
TypeError: A is base class.
>>> b = B(1)
>>> b.foo
1
>>> c = C(1, 2)
>>> c.foo, c.bar
(1, 2)
>>>

HTH
--Inyeol Lee