code: def get_leaves_by_type(graph, func=None, prune_threshold=1):
      
      for node, data in graph.nodes(data=True):
          if func and func != data.get(FUNCTION):
              continue
  
          if graph.in_degree(node) + graph.out_degree(node) <= prune_threshold:
              yield node
docs: Returns an iterable over all nodes in graph (in-place) with only a connection to one node. Useful for gene and
       RNA. Allows for optional filter by function type.
  
      :param pybel.BELGraph graph: A BEL graph
      :param func: If set, filters by the node's function from :mod:`pybel.constants` like
                      :data:`pybel.constants.GENE`, :data:`pybel.constants.RNA`,  :data:`pybel.constants.PROTEIN`, or 
                      :data:`pybel.constants.BIOPROCESS`
      :type func: str
      :param prune_threshold: Removes nodes with less than or equal to this number of connections. Defaults to :code:`1`
      :type prune_threshold: int
      :return: An iterable over nodes with only a connection to one node
      :rtype: iter[tuple]