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 GraphAdjacencyList.add_edge (graphs/graph_adjacency_list.py:93) when either the source or destination vertex has not been added to the graph. add_edge cannot create implicit vertices, so both endpoints must already exist via add_vertex (or the vertices constructor parameter).

Source

Thrown at graphs/graph_adjacency_list.py:93

            msg = f"Incorrect input: {vertex} is already in the graph."
            raise ValueError(msg)
        self.adj_list[vertex] = []

    def add_edge(self, source_vertex: T, destination_vertex: T) -> None:
        """
        Creates an edge from source vertex to destination vertex. If any
        given vertex doesn't exist or the edge already exists, 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} or "
                f"{destination_vertex} does not exist"
            )
            raise ValueError(msg)
        if self.contains_edge(source_vertex, destination_vertex):
            msg = (
                "Incorrect input: The edge already exists between "
                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.

View on GitHub (pinned to f5988cc097)

Solutions

Add both endpoints before the edge: ensure contains_vertex for both, calling add_vertex as needed
Or supply all vertices up front: GraphAdjacencyList(vertices=vs, edges=es) — but note this still requires edge endpoints to be in vs
Validate edges against the vertex set at load time and report unknown endpoints with row numbers

Example fix

# before
g = GraphAdjacencyList(vertices=["A"], edges=[])
g.add_edge("A", "B")  # raises: B does not exist

# after
g = GraphAdjacencyList(vertices=["A"], edges=[])
for v in ("A", "B"):
    if not g.contains_vertex(v):
        g.add_vertex(v)
g.add_edge("A", "B")

Defensive patterns

Strategy: validation

Validate before calling

for v in (source_vertex, destination_vertex):
    if not g.contains_vertex(v):
        g.add_vertex(v)
g.add_edge(source_vertex, destination_vertex)

Try / catch

try:
    g.add_edge(src, dst)
except ValueError as exc:
    if "does not exist" in str(exc):
        g.add_vertex(dst)
        g.add_edge(src, dst)
    else:
        raise

Prevention

Register all vertices before any edges (constructor vertices= parameter or add_vertex loop)
Cross-check edge endpoint names against the vertex list at load time

When it happens

Trigger: g = GraphAdjacencyList(vertices=['A']); g.add_edge('A', 'Z') — 'Z' missing; building edges first and vertices later; typos or case mismatches between the vertex list and edge endpoints ('alice' vs 'Alice').

Common situations: Loading vertices and edges from separate files where the edge file references vertices absent from the vertex file; incremental construction where edges are added before their endpoints; inconsistent naming from data normalization bugs.

Related errors

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/efc034daa8e26bc1. Report an issue: GitHub.