python extension problem (newbie)

Gordon McMillan gmcm at hypernet.com
Wed Jun 2 06:01:41 CEST 1999


Douglas du Boulay writes:
> 
> I've been trying to write a python extension in C to interface to a
> Fortran program and a class wrapper for the extension using the
> following code loosely adapted from Mark Lutz's Programming Python 
> stackmodule example.
> 
> Unfortunately whereas his example works fine, mine goes into a
> recursive loop whenever an instance is instantiated.  It seems that
> the __getattr__  method of the wrapper class cannot find the getattr
> method of the extension library  and defaults to trying to call
> itself, infinitely recursively.

You didn't need an extension module to do this. It's all in the 
wrapper. While __getattr__ is only called if an appropriate attribute 
is not found, __setattr__ is *always* called when trying to set an 
attribute.

So the assignment to self._base in __init__ invokes __setattr__ 
recursively. A print statement (the only one you're missing, of 
course) will prove this (and make you go boom in slow motion).

If you use __setattr__, you need to do all your assignments (in 
methods of the class like this:
    self.__dict__['_base'] = start or p2fgraspint.Grasp()

(I do this about half the time I use __setattr__. Now you'll know 
what to look for, too).

> >>file oograsp.py
> 
> import p2fgraspint
> 
> class Grasp:
>     def __init__(self, start=None):
>         self._base = start or p2fgraspint.Grasp()
>         print "doing done"                  # which is never
>         executed
>     def __getattr__(self, name):
>         print "called getattr"              # executed  endlessly
>         return getattr( self._base , name)
>     def __setattr__(self, name, value):
>         return setattr(self._base, name, value)
> 
> >> file  http://www.crystal.uwa.edu.au/~ddb/pyextend/p2fgraspint.c
> 
> >> file  graspsub.py
> 
> #!/usr/bin/env python
> import oograsp
> x = oograsp.Grasp()
> 
> 
> 

- Gordon




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