class TPySelector: public TSelector

                      Python equivalent PROOF base class


 The problem with deriving a python class from a PyROOT bound class and then
 handing it back to a C++ framework, is that the virtual function dispatching
 of C++ is completely oblivious to the methods overridden in python. To work
 within the PROOF (C++) framework, a python class should derive from the class
 TPySelector. This class provides the proper overrides on the C++ side, and
 then forwards them, as apropriate, to python.

 This is an example set of scripts:

 ### PROOF running script, very close to equivalent .C (prooftest.py)
 import time
 from ROOT import *

 dataset = TDSet( 'TTree', 'h42' )
 dataset.Add( 'root:// .... myfile.root' )

 proof = TProof.Open('')
 time.sleep(1)                     # needed for GUI to settle
 print dataset.Process( 'TPySelector', 'aapje' )
 ### EOF

 ### selector module (aapje.py, name has to match as per above)
 from ROOT import TPySelector

 class MyPySelector( TPySelector ):
    def Begin( self ):
       print 'py: beginning'

    def SlaveBegin( self, tree ):
       print 'py: slave beginning'

    def Process( self, entry ):
       if self.fChain.GetEntry( entry ) <= 0:
          return 0
       print 'py: processing', self.fChain.MyVar
       return 1

   def SlaveTerminate( self ):
       print 'py: slave terminating'

   def Terminate( self ):
       print 'py: terminating'
 ### EOF