NetworkX MultiDiGraph doesn't get printed

Question

I am trying to print an instance of NetworkX MultiDiGraph as follows:

import networkx as nx
G = nx.MultiDiGraph()
G.add_node(0)
print(G)

My understanding is that since G is an instance of a class, it should print something to the string even if __str__ is not implemented. However, this only prints a blank line. I also tested whether this is a NoneType object:

isinstance(G, nx.MultiDiGraph)

This returns True . How is this possible? If it is relevant, I am passing this graph to some other package which is returning an error related to NoneType input. Thanks in advance for any help.

Answer 1

Networkx allows you to name a graph, which is what you get when you print the graph. So for example:

import networkx as nx
G = nx.MultiDiGraph(name = 'myG')
G.add_node(0)
print(G)
> myG

The name defaults to an empty string. So that is what is being printed. I think it's not obvious from the code, but at this link , the definition of the Graph class is given. In it, __str__ is defined to return the graph name. The graph name is defined as

@property
    def name(self):
        """String identifier of the graph.

        This graph attribute appears in the attribute dict G.graph
        keyed by the string `"name"`. as well as an attribute (technically
        a property) `G.name`. This is entirely user controlled.
        """
        return self.graph.get("name", "")

So the default name is an empty string. Now you've used a MultiDiGraph , so this is hidden away - a MultiDiGraph is built on a DiGraph , which is built on a Graph , so hidden in all of that, if the MultiDiGraph isn't given a name, it defaults to an empty string.