from AppKit import *
from PyObjCTools import AppHelper

class MyFormatter(NSFormatter):
    def stringForObjectValue_(self, product):
        if isinstance(product, (str, unicode)):
                return product
        return str(product)

    def getObjectValue_forString_errorDescription_(self, value, upc, error):
        print self, upc

        if not upc:
            print "No data"
            return True, None, None

        print "Have data"
        return False, None, NSString.stringWithString_("Foo the %s"%("bar",))

AppHelper.runEventLoop()