TheAlgorithms/Python · error · ValueError

Incorrect input: {vertex} does not exist in this graph.

Error message

Incorrect input: {vertex} does not exist in this graph.

What it means

Raised by GraphAdjacencyList.remove_vertex when the vertex passed is not a key in the internal adjacency dict (checked via contains_vertex, i.e. `vertex in self.adj_list`). The library refuses to remove something that was never added so the dict stays consistent. It is a plain ValueError, so it can be caught with `except ValueError`.

Source

Thrown at graphs/graph_adjacency_list.py:115

                f"{source_vertex} and {destination_vertex}"
            )
            raise ValueError(msg)

        # add the destination vertex to the list associated with the source vertex
        # and vice versa if not directed
        self.adj_list[source_vertex].append(destination_vertex)
        if not self.directed:
            self.adj_list[destination_vertex].append(source_vertex)

    def remove_vertex(self, vertex: T) -> None:
        """
        Removes the given vertex from the graph and deletes all incoming and
        outgoing edges from the given vertex as well. If the given vertex
        does not exist, a ValueError will be thrown.
        """
        if not self.contains_vertex(vertex):
            msg = f"Incorrect input: {vertex} does not exist in this graph."
            raise ValueError(msg)

        if not self.directed:
            # If not directed, find all neighboring vertices and delete all references
            # of edges connecting to the given vertex
            for neighbor in self.adj_list[vertex]:
                self.adj_list[neighbor].remove(vertex)
        else:
            # If directed, search all neighbors of all vertices and delete all
            # references of edges connecting to the given vertex
            for edge_list in self.adj_list.values():
                if vertex in edge_list:
                    edge_list.remove(vertex)

        # Finally, delete the given vertex and all of its outgoing edge references
        self.adj_list.pop(vertex)

    def remove_edge(self, source_vertex: T, destination_vertex: T) -> None:
        """

View on GitHub (pinned to f5988cc097)

Solutions

1. Check `graph.contains_vertex(vertex)` before calling remove_vertex.
2. Audit the calling loop for double-removal or stale vertex references (e.g. iterating a snapshot while mutating).
3. Verify the vertex type matches what was used in add_vertex (1 vs '1').
4. Wrap the call in try/except ValueError if removal is best-effort (e.g. cleanup paths).

Example fix

// before
graph.remove_vertex(node)  # node may already be gone

// after
if graph.contains_vertex(node):
    graph.remove_vertex(node)

if graph.contains_vertex(vertex):
    graph.remove_vertex(vertex)

def can_remove_vertex(graph, vertex) -> bool:
    return graph.contains_vertex(vertex)

try:
    graph.remove_vertex(vertex)
except ValueError as exc:
    if "does not exist in this graph" not in str(exc):
        raise
    logger.debug("vertex already absent: %r", vertex)

When it happens

Trigger: Calling remove_vertex(v) on a graph that never had v added, after v was already removed, or after clear_graph(). Also triggered when the vertex object is equal-but-different in a way the dict does not match (e.g. passing 1 vs '1', or an unhashable type raising before this check).

Common situations: Processing user-supplied node lists where some nodes were filtered out before graph construction; double-removal in loops; calling remove_vertex inside an iteration that already deleted the vertex; confusion between value types (str vs int vertex labels).

Related errors

• Incorrect input: Either {source_vertex} or {destination_vert
• Incorrect input: {vertex} does not exist in this graph.
• Power cannot be negative in any electrical/electronics syste
• One and only one argument must be 0
• One and only one argument must be 0

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/74d648cc54e1f567. Report an issue: GitHub.