[Python-Dev] Evil setattr hack

Phillip J. Eby pje@telecommunity.com
Tue, 15 Apr 2003 12:45:36 -0400

 >I've checked in what I believe is an adequate block for at least this
 >particular hack.  wrap_setattr(), which is called in response to
 ><type>.__setattr__(), now compares if the C function it is about to
 >call is the same as the C function in the built-in base class closest
 >to the object's class.  This means that if B is a built-in class and P
 >is a Python class derived from B, P.__setattr__ can call
 >B.__setattr__, but not A.__setattr__ where A is an (also built-in)
 >base class of B (unless B inherits A.__setattr__).

Does this follow __mro__ or __base__?  I'm specifically wondering about the 
implications of multiple inheritance from more than one C base class; this 
sort of thing (safety checks relating to heap vs. non-heap types and the 
"closest" method of a particular kind) has bitten me before in relation to 
ZODB4's Persistence package.  In that context, mixing 'type' and 
'PersistentMetaClass' makes it impossible to instantiate the resulting 
metaclass, because neither type.__new__ nor PersistentMetaClass.__new__ is 
considered "safe" to execute.  My "evil hack" to fix that was to add an 
extra PyObject * to PersistentMetaClass so that it has a larger 
tp_basicsize than 'type' and Python then considers it the '__base__' type, 
thus causing its '__new__' method to be accepted as legitimate.