[XML-SIG] Wrapping a DOM class.

[Fred L. Drake, Jr.]

Ian Sparks writes:
 > I confess the XML support for Python confuses me. I thought I understood the
 > DOM but I'm having problems with the Python DOM implementations. The
 > following sample shows what I am *trying* to do. I know the minidom won't
 > get me here so what implementation will? If someone could convert to a
 > working sample I'd be very grateful.

  minidom will work just fine; the problem you're having has nothing
to do with the DOM.
  What's happening is that you are defining an attribute for xdoc
instances named "dom"; that's a class object, and has no
documentElement attribute itself.  You can get rid of the "dom =
Document" assignment in the class definition.
  Once that's out of the way, the constructor will attempt to create
the "dom" attribute, but that will go through the __setattr__() hook,
which then expects to find the not-yet-created "dom" attribute, so the
__getattr__() is called with the name "dom".  That then uses the
not-yet-created "dom" attribute to get the document object, causing a
recursive call to __getattr__() -- this will eventually fail.
  If you change the constructor to avoid the __setattr__(), the rest
of the class needs no change:

------------------------------------------------------------------------
class xdoc:
    def __init__(self):
        self.__dict__['dom'] = parseString('<myxml att="hello"/>')

    def __getattr__(self,key):
        return self.dom.documentElement.getAttributeNS('',key)
    
    def __setattr__(self,key,value):
        self.dom.documentElement.setAttributeNS('',key,value)
------------------------------------------------------------------------

  And it works now, to boot!


  -Fred

-- 
Fred L. Drake, Jr.  <fdrake at acm.org>
PythonLabs at Digital Creations