Subgraph isomorphism fails with parallel edges

[georgios-ts]

Information

• retworkx version: main
• Python version: Any
• Rust version: Any
• Operating system: Any

What is the current behavior?

is_subgraph_isomorphic returns True even in cases it shouldn't. This happens if the input "small" graph have parallel edges.

What is the expected behavior?

Handle correctly graphs with parallel edges.

Steps to reproduce the problem

import retworkx as rx

first = rx.PyGraph()
first.extend_from_edge_list([
    (0, 1), (1, 2), (2, 3)
])
second = rx.PyGraph()
second.extend_from_edge_list([
    (0, 1), (0, 1)
])
rx.is_subgraph_isomorphic(first, second)
---
True

[mtreinish]

Fixing this definitely seems worth including in a 0.10.2 release.

[georgios-ts]

UPDATE:
Even retworkx.is_isomorphic fails with parallel edges if an edge_matcher is specified.
An example:

import retworkx as rx

graph = rx.PyGraph()
graph.extend_from_weighted_edge_list([
    (0, 1, 'a'), (0, 1, 'b'), (1, 2, 'c')
])

rx.is_isomorphic(graph, graph,
                 edge_matcher=lambda x, y: x == y)
----
False

UPDATE 2:
retworkx.is_subgraph_isomorphic may panic if induced=False and an edge_matcher is specified.

first = rx.PyGraph()
first.extend_from_weighted_edge_list([
    (0, 1, 'a'), (1, 2, 'b'), (2, 0, 'c')
])

second = rx.PyGraph()
second.extend_from_weighted_edge_list([
    (0, 1, 'a'), (1, 2, 'b')
])

rx.is_subgraph_isomorphic(first, second, induced=False,
                          edge_matcher=lambda x, y: x == y
)
---
PanicException: internal error: entered unreachable code