Graph Union in Python Igraph

Question

I have 3 subgraphs and I want to obtain their union using the Python package Igraph (v. 0.9.1). I use Python v. 3.8.5.

Let's assume a simple example of 3 directed graphs of 5, 4 and 8 vertices respectively. Each vertex has an attribute called "name" (I give each one a unique String label).

# Note: igraph version: 0.9.1
from igraph import *

# Subgraphs

# g1: 5 vertices and 3 edges
g1 = Graph(directed=True)
g1.add_vertices(5)
g1.vs["name"] = ["1_g1", "2_g1", "3_g1", "4_g1", "5_g1"]
g1.add_edges([(0, 1), (2, 3), (4, 2)])

# g2: 4 vertices and 4 edges
g2 = Graph(directed=True)
g2.add_vertices(4)
g2.vs["name"] = ["1_g2", "2_g2", "3_g2", "4_g2"]
g2.add_edges([(1, 2), (3, 1), (2, 3), (0, 1)])

# g3: 3 vertices and 2 edges
g3 = Graph(directed=True)
g3.add_vertices(3)
g3.vs["name"] = ["1_g3", "2_g3", "3_g3"]
g3.add_edges([(0, 2), (1, 0)])

# Union of the 3 subgraphs
g_union = Graph(directed=True)

# Doesn't give me the result I expect...
g_union = g_union.union([g1, g2, g3], byname=True)

If I have 3 graphs that do not share edges and do not share the value of the attribute "name", I expect the union to have 12 vertices (5 + 4 + 3) and 9 edges (3 + 4 + 2). This is the graph I want to obtain. However, I get an error "AttributeError: Some graphs are not named" .

The "union" method has 2 parameters: "graphs", "byname" (default value: 'auto'). Here is the reference: https://igraph.org/python/doc/api/igraph.operators.html

Answer 1

I found the answer, instead of the "union" method I should have used the "disjoint_union" method (whose parameter is the list of graphs to combine). This function keeps the attributes of all graphs.

All graph, vertex and edge attributes are copied to the result.