is_tree#

Returns True if G is a tree.

A tree is a connected graph with no undirected cycles.

For directed graphs, G is a tree if the underlying graph is a tree. The underlying graph is obtained by treating each directed edge as a single undirected edge in a multigraph.

Parameters:

Ggraph: The graph to test.

Returns:

bbool: A boolean that is True if G is a tree.

Raises:

NetworkXPointlessConcept: If G is empty.

See also

is_arborescence

Notes

In another convention, a directed tree is known as a polytree and then tree corresponds to an arborescence.

Examples

>>> G = nx.Graph()
>>> G.add_edges_from([(1, 2), (1, 3), (2, 4), (2, 5)])
>>> nx.is_tree(G)  # n-1 edges
True
>>> G.add_edge(3, 4)
>>> nx.is_tree(G)  # n edges
False