TheAlgorithms/Python · error · ValueError

Incorrect input: {vertex} is already in the graph.

Error message

Incorrect input: {vertex} is already in the graph.

What it means

Raised by GraphAdjacencyList.add_vertex (graphs/graph_adjacency_list.py:76) when the vertex is already present (contains_vertex returns True). Vertices are unique keys in the adjacency dict, so adding a duplicate is treated as an incorrect-input error rather than an idempotent no-op.

Source

Thrown at graphs/graph_adjacency_list.py:76

            self.add_edge(edge[0], edge[1])

    def add_vertex(self, vertex: T) -> None:
        """
        Adds a vertex to the graph. If the given vertex already exists,
        a ValueError will be thrown.

        >>> g = GraphAdjacencyList(vertices=[], edges=[], directed=False)
        >>> g.add_vertex("A")
        >>> g.adj_list
        {'A': []}
        >>> g.add_vertex("A")
        Traceback (most recent call last):
        ...
        ValueError: Incorrect input: A is already in the graph.
        """
        if self.contains_vertex(vertex):
            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):

View on GitHub (pinned to f5988cc097)

Solutions

  1. Guard with contains_vertex: if not g.contains_vertex(v): g.add_vertex(v)
  2. Deduplicate the input before constructing: vertices = list(set(vertices)) (order-insensitive) or dict.fromkeys(vertices) to preserve order
  3. Let the constructor register vertices from edges by calling add_edge, which only requires vertices added beforehand — or just catch ValueError where duplicates are benign

Example fix

# before
for v in vertex_list:
    g.add_vertex(v)  # raises on duplicates

# after
for v in dict.fromkeys(vertex_list):  # deduped, order preserved
    if not g.contains_vertex(v):
        g.add_vertex(v)
Defensive patterns

Strategy: validation

Validate before calling

for v in dict.fromkeys(vertices):  # dedupe, keep order
    if not g.contains_vertex(v):
        g.add_vertex(v)

Try / catch

try:
    g.add_vertex(v)
except ValueError:
    pass  # already present; treat as idempotent

Prevention

When it happens

Trigger: g.add_vertex("A") twice; building a graph from a list that mentions a vertex in several edges, then adding all edge endpoints with add_vertex in a loop; case-sensitive vertex sets where 'a' and 'A' are distinct but data contains near-duplicates.

Common situations: Loading vertex lists that contain duplicates; re-running ingestion code against the same graph object; user data where the same entity appears under the same label multiple times.

Related errors

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/a32dfa9fee17b6c4. Report an issue: GitHub.