[PEAK] Type and class adaptation

[Phillip J. Eby]

On my second reading, I caught a couple of things that I missed before...


At 06:21 PM 11/19/04 +0100, Radek Kanovsky wrote:
>Additionally if I declare
>adapter for MyClass, this adapter is not applicable to SubClass.

This doesn't sound right; do you have a short code sample I could use to 
test this?




>def dispatch_by_subclass (klass, table):
>     while True:
>         if klass in table:
>             return table[klass]
>         try:
>             klass, = klass.__bases__

This code will raise an AttributeError if it is used on a non-class.


>         except ValueError:
>             if klass.__bases__:
>                 return table.reseed(klass)
>             else:
>                 break
>     if klass in table:
>         return table[object]

This last bit is also broken; it's *always* going to return None.

Essentially, the problem with your implementation as far as I can tell, is 
that it doesn't at all consider the possibility that the object is not a 
class.  I would recommend that you use 'None' as the key for that 
situation.  So, the code would look something like this:

def dispatch_by_subclass (klass, table):
     if isinstance(klass,ClassTypes):
         while True:
             if klass in table:
                 return table[klass]
             try:
                 klass, = klass.__bases__
             except ValueError:
                 if klass.__bases__:
                     return table.reseed(klass)
                 else:
                     break
     if None in table:
         return table[None]

And, the 'ISSubclass.seeds()' method should also include 'None' in its seeds.

Essentially, a dispatch function must never return 'None' unless there are 
no cases registered for that situation, and an ITest must create seeds for 
all the possibilities  regarding that test.  One of the possibilities of a 
subclass test is that the object being tested is not a class, so there 
needs to be a key for that condition.  Also, there is the possibility that 
a class is not a subclass of any known test (i.e., it's a new classic 
class).  In both of these cases, the 'None' key represents the default 
dispatch path, which is basically a path where none of the subclass-testing 
alternatives are applicable.

This is different from dispatch_by_mro, because in Python everything is an 
*instance* of 'object' eventually, so 'InstanceType' and 'object' are the 
default keys.  But, in this case, not everything is a *subclass* of 
'object', so a different default is needed.