Overriding special methods on an instance

Question

I hope someone can answer this that has a good deep understanding of Python :)

Consider the following code:

>>> class A(object):
...     pas         


        

Answer 1

For new style classes, Python uses a special method lookup that bypasses instances. Here an excerpt from the source:

  1164 /* Internal routines to do a method lookup in the type
  1165    without looking in the instance dictionary
  1166    (so we can't use PyObject_GetAttr) but still binding
  1167    it to the instance.  The arguments are the object,
  1168    the method name as a C string, and the address of a
  1169    static variable used to cache the interned Python string.
  1170 
  1171    Two variants:
  1172 
  1173    - lookup_maybe() returns NULL without raising an exception
  1174      when the _PyType_Lookup() call fails;
  1175 
  1176    - lookup_method() always raises an exception upon errors.
  1177 
  1178    - _PyObject_LookupSpecial() exported for the benefit of other places.
  1179 */

You can either change to an old-style class (don't inherit from object) or you can add dispatcher methods to the class (methods that forward lookups back to the instance). For an example of instance dispatcher methods, see the recipe at http://code.activestate.com/recipes/578091