User dictionary via ** kw

Question

I have a library function that uses **kw, but I want to pass a dictionary-like class so that I can override __getitem__to track its access to data in the dictionary. For example, in the code below, calling libfn does not print Accessed, but libfn2 does.

class Dtracker(dict):
  def __init__(self):
    dict.__init__(self)
  def __getitem__(self,item):
    print "Accessed %s" % str(item)
    return dict.__getitem__(self, item)


def libfn(**kw):
  a = kw["foo"]
  print "a is %s" % a
  return a

def libfn2(kw):
  a = kw["foo"]
  print "a is %s" % a
  return a

d = Dtracker()
d["foo"] = "bar"  
libfn(**d)
libfn2(d)

Answer 1

Do it

class Dtracker(dict):
    def __init__(self,*arg,**kw):
        super(Dtracker,self).__init__(*arg,**kw)
    def __getitem__(self,item):
        print "Accessed %s" % str(item)
        return dict.__getitem__(self, item)

def track( fn ):
    def tracked_fn( **kw ):
        kw= Dtracker( kw )
        fn( kw )
    return tracked_fn

@track
def libfn(kw):
    a = kw["foo"]
    print "a is %s" % a
    return a

It more or less works

>>> libfn( **{'foo':'bar'} )
Accessed foo
a is bar

Answer 2

You cannot, without changing Python itself. It is converted dictto a lower level.