TheAlgorithms/Python · error · ValueError

Incorrect input: Either {source_vertex} or {destination_vert

Error message

Incorrect input: Either {source_vertex} or {destination_vertex} does not exist.

What it means

Raised by GraphAdjacencyMatrix.contains_edge when either queried endpoint is not a registered vertex. Like the adjacency-list counterpart, this predicate validates its inputs and raises instead of returning False for unknown vertices, because it must map both endpoints to matrix indices (vertex_to_index lookups) to read the cell.

Source

Thrown at graphs/graph_adjacency_matrix.py:185

        Returns True if the graph contains the vertex, False otherwise.
        """
        return vertex in self.vertex_to_index

    def contains_edge(self, source_vertex: T, destination_vertex: T) -> bool:
        """
        Returns True if the graph contains the edge from the source_vertex to the
        destination_vertex, False otherwise. If any given vertex doesn't exist, a
        ValueError will be thrown.
        """
        if not (
            self.contains_vertex(source_vertex)
            and self.contains_vertex(destination_vertex)
        ):
            msg = (
                f"Incorrect input: Either {source_vertex} "
                f"or {destination_vertex} does not exist."
            )
            raise ValueError(msg)

        u = self.vertex_to_index[source_vertex]
        v = self.vertex_to_index[destination_vertex]
        return self.adj_matrix[u][v] == 1

    def clear_graph(self) -> None:
        """
        Clears all vertices and edges.
        """
        self.vertex_to_index = {}
        self.adj_matrix = []

    def __repr__(self) -> str:
        first = "Adj Matrix:\n" + pformat(self.adj_matrix)
        second = "\nVertex to index mapping:\n" + pformat(self.vertex_to_index)
        return first + second

View on GitHub (pinned to f5988cc097)

Solutions

Check both endpoints first: `graph.contains_vertex(u) and graph.contains_vertex(v) and graph.contains_edge(u, v)`.
Restrict edge queries to ids drawn from the graph's own vertex set.
Catch ValueError when scanning pairs where unknown vertices are expected.

Example fix

# before
if graph.contains_edge(u, v):  # raises on unknown vertex
    ...

# after
if (
    graph.contains_vertex(u)
    and graph.contains_vertex(v)
    and graph.contains_edge(u, v)
):
    ...

Defensive patterns

Strategy: validation

Validate before calling

def safe_contains_edge(graph, u, v) -> bool:
    if not (graph.contains_vertex(u) and graph.contains_vertex(v)):
        return False
    return graph.contains_edge(u, v)

Type guard

def safe_contains_edge(graph, u, v) -> bool:
    if not (graph.contains_vertex(u) and graph.contains_vertex(v)):
        return False
    return graph.contains_edge(u, v)

Try / catch

try:
    has = graph.contains_edge(u, v)
except ValueError:
    has = False

Prevention

Treat contains_edge as raising on unknown vertices, not as a total function.
Filter candidate pairs by the graph's vertex set before probing.
Keep a helper (safe_contains_edge) and use it everywhere instead of raw calls.

When it happens

Trigger: Calling contains_edge(u, v) where u or v is not in vertex_to_index; probing candidate edges against a graph built from a subset of nodes; querying after remove_vertex or clear_graph removed an endpoint.

Common situations: Using contains_edge as a filter over arbitrary node pairs from external data; assuming predicates are exception-safe in validation code; checking for an edge whose endpoints were never verified to exist.

Related errors

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/d52ecd4240f48c4c. Report an issue: GitHub.