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fix(graph): support edge lookup via tuple key in GenericGraph.__getitem__ #4753

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fix(graph): support edge lookup via tuple key in GenericGraph.__getitem__ #4753

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2 changes: 2 additions & 0 deletions manim/mobject/graph.py
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Original file line number Diff line number Diff line change
Expand Up @@ -670,6 +670,8 @@ def _populate_edge_dict(
raise NotImplementedError("To be implemented in concrete subclasses")

def __getitem__(self: Graph, v: Hashable) -> Mobject:
if isinstance(v, tuple) and len(v) == 2:
return self.edges[v]
return self.vertices[v]
Comment on lines 672 to 675
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@chopan050 chopan050 Jun 2, 2026

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One thing I mentioned in issue #3798 is that vertices can be any hashable object. This includes tuples: vertices can be tuples themselves. This is not too strange, actually. For example, vertices could represent 2D coordinates. There's also this concept of a "line graph L of a graph G" where each vertex of L is an edge of G: if there is an edge connecting vertices 1 and 3 of G, then L has a vertex (1, 3), and so on. networkx actually implements this.

It is possible that the graph contains two vertices, say, (1, 2) and (1, 3), and an edge connecting both: ((1, 2), (1, 3)). Thus, verifying that v is a tuple containing two elements is not enough to determine whether it represents an edge or a vertex.

One way I would solve this is by directly asking whether v is contained in the vertices or the edges before attempting to retrieve the Mobject:

if v in self.vertices:
    return self.vertices[v]
if v in self.edges:
    return self.edges[v]
raise IndexError(f"Vertex or edge {v} not found")
# or some more elaborated message, like
# "Couldn't find vertex {v} or edge connecting vertices {v[0]} and {v[1]}"
# when v is a tuple of 2 elements


def _create_vertex(
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13 changes: 13 additions & 0 deletions tests/module/mobject/test_graph.py
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Original file line number Diff line number Diff line change
Expand Up @@ -76,6 +76,19 @@ def test_graph_add_edges():
assert set(G._graph.edges()) == set(G.edges.keys())


def test_graph_getitem():
vertices = [1, 2, 3, 4]
edges = [(1, 2), (2, 3), (3, 4), (4, 1)]
G = Graph(vertices, edges)
# Vertex access
assert G[1] is G.vertices[1]
# Edge access via tuple key
assert G[(1, 2)] is G.edges[(1, 2)]
# DiGraph edge access
DG = DiGraph(vertices, edges)
assert DG[(1, 2)] is DG.edges[(1, 2)]


def test_graph_remove_edges():
G = Graph([1, 2, 3, 4, 5], [(1, 2), (2, 3), (3, 4), (4, 5), (1, 5)])
removed_mobjects = G.remove_edges((1, 2))
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