code: def set_sort_function(sortable, callback, column=0):
      
      sortable.set_default_sort_func(
          lambda tree, itera, iterb: _normalize(callback(
              tree.get_value(itera, column),
              tree.get_value(iterb, column)
              ))
          )
docs: *sortable* is a :class:`gtk.TreeSortable` instance.
  
      *callback* will be passed two items and must return a value like
      the built-in `cmp`.
  
      *column* is an integer adressing the column that holds items.
  
      This will re-sort even if *callback* is the same as before.
  
      .. note::
         When sorting a `ListStore` without a `TreeModelSort` you have to call
         `set_sort_column_id(-1, gtk.SORT_ASCENDING)` once, *after* this.