[C++-sig] hijacking getattr/setattr

Václav Šmilauer eudoxos at arcig.cz
Thu May 7 19:21:21 CEST 2009


I have  wrapper c++ classes that are exposed to python that wrap "real" 
c++ classes with data. Wrapper classes have some attributes that .def'ed 
and .add_property'd in the glue code. Wrapped classes have also 
attributes that are registered by name (for serialization purposes) and 
I can retrieve them.

To be able to get both wrapper attributes and attributes of the wrapped 
class, I use getitem/setitem for the latter ones. That makes the python 
code a bit ugly and difficult to read:

  b.geom['radius']=5  ## .geom returns wrapped class holding geometry 
attributes, is .add_property'ed
  b.phys['mass']=8     ## .phys returns wrapped class holding physical 
attributes, is .add_property'ed
  print b['id']           ## get an attribute that exists as such in the 
wrapped class

I would like to unify both approaches so that if I say b.id, the pyBody 
object (wrapping a shared_ptr<Body> object) will first look if the 
attribute 'id' is within attributes of the wrapper class; return it if 
it is, look in the wrapped object attributes if it is not; return the 
wrapped object attribute of that name if it exists, otherwise throw. 
(The same for b.geom.radius, b.phys.mass)

I simulated that with pure python code (attached), but it is not clear 
to me how to put this in boost::python.

Can I get some pointers? I can overrider __getattr__ and __setattr__ of 
the wrapper class, but I don't know how to simulate the default behavior 
of getattr/setattr, once the key wasn't found in the wrapped class, for 



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