'Specify a function COMMAND to call if the X
selection owned by this widget is queried by another
application.
This function must return the contents of the
selection. The function will be called with the
arguments OFFSET and LENGTH which allows the chunking
of very long selections. The following keyword
parameters can be provided:
selection - name of the selection (default PRIMARY),
type - type of the selection (e.g. STRING, FILE_NAME).'

def selection_handle(self, command, **kw):

    name = self._register(command)
    self.tk.call(((('selection', 'handle') + self._options(kw)) + (self._w, name)))
