code: def construct_graph_from_formula(
      cls, formula=None, attr="name", simplify=True
  ):
      """Graph.Formula(formula = None, attr = "name", simplify = True)
  
      Generates a graph from a graph formula
  
      A graph formula is a simple string representation of a graph.
      It is very handy for creating small graphs quickly. The string
      consists of vertex names separated by edge operators. An edge
      operator is a sequence of dashes (C{-}) that may or may not
      start with an arrowhead (C{<} at the beginning of the sequence
      or C{>} at the end of the sequence). The edge operators can
      be arbitrarily long, i.e., you may use as many dashes to draw
      them as you like. This makes a total of four different edge
      operators:
  
        - C{-----} makes an undirected edge
        - C{<----} makes a directed edge pointing from the vertex
          on the right hand side of the operator to the vertex on
          the left hand side
        - C{---->} is the opposite of C{<----}
        - C{<--->} creates a mutual directed edge pair between
          the two vertices
  
      If you only use the undirected edge operator (C{-----}),
      the graph will be undirected. Otherwise it will be directed.
      Vertex names used in the formula will be assigned to the
      C{name} vertex attribute of the graph.
  
      Some simple examples:
  
        >>> from igraph import Graph
        >>> print(Graph.Formula())          # empty graph
        IGRAPH UN-- 0 0 --
        + attr: name (v)
        >>> g = Graph.Formula("A-B")        # undirected graph
        >>> g.vs["name"]
        ['A', 'B']
        >>> print(g)
        IGRAPH UN-- 2 1 --
        + attr: name (v)
        + edges (vertex names):
        A--B
        >>> g.get_edgelist()
        [(0, 1)]
        >>> g2 = Graph.Formula("A-----------B")
        >>> g2.isomorphic(g)
        True
        >>> g = Graph.Formula("A  --->  B") # directed graph
        >>> g.vs["name"]
        ['A', 'B']
        >>> print(g)
        IGRAPH DN-- 2 1 --
        + attr: name (v)
        + edges (vertex names):
        A->B
  
      If you have may disconnected componnets, you can separate them
      with commas. You can also specify isolated vertices:
  
        >>> g = Graph.Formula("A--B, C--D, E--F, G--H, I, J, K")
        >>> print(", ".join(g.vs["name"]))
        A, B, C, D, E, F, G, H, I, J, K
        >>> g.connected_components().membership
        [0, 0, 1, 1, 2, 2, 3, 3, 4, 5, 6]
  
      The colon (C{:}) operator can be used to specify vertex sets.
      If an edge operator connects two vertex sets, then every vertex
      from the first vertex set will be connected to every vertex in
      the second set:
  
        >>> g = Graph.Formula("A:B:C:D --- E:F:G")
        >>> g.isomorphic(Graph.Full_Bipartite(4, 3))
        True
  
      Note that you have to quote vertex names if they include spaces
      or special characters:
  
        >>> g = Graph.Formula('"this is" +- "a silly" -+ "graph here"')
        >>> g.vs["name"]
        ['this is', 'a silly', 'graph here']
  
      @param formula: the formula itself
      @param attr: name of the vertex attribute where the vertex names
                   will be stored
      @param simplify: whether the simplify the constructed graph
      @return: the constructed graph:
      """
  
      # If we have no formula, return an empty graph
      if formula is None:
          return cls(0, vertex_attrs={attr: []})
  
      vertex_ids, edges, directed = UniqueIdGenerator(), [], False
      # Loop over each part in the formula
      for part in re.compile(r"[,\n]").split(formula):
          # Strip leading and trailing whitespace in the part
          part = part.strip()
          # Parse the first vertex specification from the formula
          for start_names, end_names, arrowheads in generate_edges(
              part
          ):
              start_ids = [vertex_ids[name] for name in start_names]
              end_ids = [vertex_ids[name] for name in end_names]
              if not arrowheads[0] and not arrowheads[1]:
                  # This is an undirected edge. Do we have a directed graph?
                  if not directed:
                      # Nope, add the edge
                      edges.extend(
                          (id1, id2)
                          for id1 in start_ids
                          for id2 in end_ids
                      )
              else:
                  # This is a directed edge
                  directed = True
                  if arrowheads[1]:
                      edges.extend(
                          (id1, id2)
                          for id1 in start_ids
                          for id2 in end_ids
                      )
                  if arrowheads[0]:
                      edges.extend(
                          (id2, id1)
                          for id1 in start_ids
                          for id2 in end_ids
                      )
  
      # Grab the vertex names into a list
      vertex_attrs = {}
      vertex_attrs[attr] = list(vertex_ids.values())
      # Construct and return the graph
      result = cls(
          len(vertex_ids), edges, directed, vertex_attrs=vertex_attrs
      )
      if simplify:
          result.simplify()
      return result
  
imports: from io import StringIO
  from igraph.datatypes import UniqueIdGenerator
  import re
  import tokenize
  import token
label_desc: Use image node_id 2 for calling a global function with example usage: construct_graph_from_formula(cls, formula, attr, simplify) and returns: result, cls