Groups of nodes with the same attributes in GraphViz file - grouping

Question

In the dot language of GraphViz I want to describe a 2-mode network. So I have nodes of two different types. One group for example could contains people how read and the other group could contain the books being read by the people.
I want to give the nodes in these two groups different looks (shape, color, etc). How can I specify the attributes for a group of nodes in one statement. The aim is to be able to change the look for each group of nodes in one place, not in all individual node descriptions.
This could be done with something like attribute inheritance, but I don't know whether the dot language has this concept.

Answer 1

In principal there are three possibilities
set default attributes before creating a node
globally - valid for all following node creations
locally in a subgraph - valid for node creations within subgraph only
create nodes with explicit attributes
assign attributes to a group of nodes after creation.
Options 1 and 2 allow only one group per node as creation is a single event. Option 3 allows different grouping for each assignment.
set default attributes globally before creating a node
digraph {
x // node with current defaults
// set default
node [shape=box color=red]
// create with default values
a1, a2
// set default
node [shape=circle color=blue]
// create with default values
b1, b2
y // node with current defaults
x->{a1 a2}
a1->{b1 b2}
a2->{b1 b2}
{b1,b2}->y
}
set default attributes locally before creating a node
digraph {
x // node with current defaults
{
// set default
node [shape=box color=red]
// create with default values
a1, a2
}
{
// set default
node [shape=circle color=blue]
// create with default values
b1, b2
}
y // node with current defaults
x->{a1 a2}
a1->{b1 b2}
a2->{b1 b2}
{b1,b2}->y
}
create nodes with explicit attributes
digraph {
x // node with current defaults
// create with explicit attributes
a1, a2 [shape=box color=red]
// create with explicit attributes
b1, b2 [shape=circle color=blue]
y // node with current defaults
x->{a1 a2}
a1->{b1 b2}
a2->{b1 b2}
{b1,b2}->y
}
assign attributes to a group of nodes after creation
digraph {
x // node with current defaults
// create with default values
a1, a2, b1, b2
// assign shape
a1, a2 [shape=box]
b1, b2 [shape=circle]
// assign color
a1, b2 [color=red]
b1, a2 [color=blue]
y // node with current defaults
x->{a1 a2}
a1->{b1 b2}
a2->{b1 b2}
{b1,b2}->y
}

Answer 2

This can be done for all nodes in a graph with the node keyword, or for all edges in graph with the edge keyword. This can also be done on a node-by-node or edge-by-edge basis.
Example for a whole graph or subgraph:
digraph
{
subgraph readers
{
node[shape=box; color=red;]
r1; r2; r3;
}
subgraph books
{
node[shape=circle; color=blue;]
b1; b2; b3;
}
r1->{b1 b2}
r2->{b2 b3}
r3->{b1 b2 b3}
}
This would give you the graph:
Example of per node attributes:
digraph
{
n1[shape=triangle];
n2[shape=star];
n3[shape=square];
n1->n2->n3
}
Would give the graph: