docs: def filter_from_mapping(self, mapping, backend=None):
          """
          Return mappings that either exactly correspond to the given `mapping`
          tuple, or, if the second item of `mapping` is `None`, include mappings
          that only match the first item of `mapping` (useful to show all
          mappings for a given project).
          """
          def mapping_filter(key_item):
              key, item = key_item
  
              return (
                  (mapping is None or item.mapping == mapping or
                      (mapping[1] is None and item.mapping is not None and item.mapping[0] == mapping[0])) and
                  (backend is None or item.backend == backend)
              )
  
          items = [item for item in six.iteritems(self) if mapping_filter(item)]
  
          aliases = collections.OrderedDict(
              sorted(items, key=lambda alias: alias[1].mapping
                     if alias[1] is not None else (0, 0))
          )
  
          return aliases