Directed networks

Sometimes, it helps to add just a little more detail to a network. The edges we've seen so far don't have any sense of coming from or going to; they are simply connections between two nodes, so they are called symmetric or undirected.

Imagine a network that represents a system of roads (edges) and intersections (nodes). A network with undirected edges would be a good representation, until you came across a one-way street. An undirected edge suggests that you can travel in either direction equally well, while in reality, driving against traffic is likely to be a rather different experience from driving with it.

When direction matters, a network is called directed. In a directed network, each edge has a source node and a target node. Typically, the edge represents a flow of some kind, for example, traffic, from the source to the target. But what if not all connections are one-way? Easy! Two-way connections are made by combining two directed edges going in opposite directions. In directed networks, edges are drawn with arrows pointing toward the target, as shown in the following diagram. In NetworkX, directed networks are represented by the DiGraph class, also described in Chapter 2, Working with Networks in NetworkX: