(cls, formula=None, attr="name", simplify=True)
return result
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.clusters().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:
Graph.Formula(formula = None, attr = "name", simplify = True)
Graph . Formula ( formula = None attr = name simplify = True )
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.clusters().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
def construct_graph_from_formula ( cls , formula = None , attr = "name" , simplify = True ) : # 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