<h2>Editing File: /usr/lib/python3.6/site-packages/ipapython/graph.py</h2><form method='post'><textarea name='content' class='file-edit'>#
# Copyright (C) 2015-2017  FreeIPA Contributors see COPYING for license
#
from collections import deque


class Graph:
    &quot;&quot;&quot;
    Simple oriented graph structure

    G = (V, E) where G is graph, V set of vertices and E list of edges.
    E = (tail, head) where tail and head are vertices
    &quot;&quot;&quot;

    def __init__(self):
        self.vertices = set()
        self.edges = []
        self._adj = dict()

    def add_vertex(self, vertex):
        self.vertices.add(vertex)
        self._adj[vertex] = []

    def add_edge(self, tail, head):
        if tail not in self.vertices:
            raise ValueError(&quot;tail is not a vertex&quot;)

        if head not in self.vertices:
            raise ValueError(&quot;head is not a vertex&quot;)

        self.edges.append((tail, head))
        self._adj[tail].append(head)

    def remove_edge(self, tail, head):
        try:
            self.edges.remove((tail, head))
        except KeyError:
            raise ValueError(
                &quot;graph does not contain edge: ({0}, {1})&quot;.format(tail, head)
            )
        self._adj[tail].remove(head)

    def remove_vertex(self, vertex):
        try:
            self.vertices.remove(vertex)
        except KeyError:
            raise ValueError(
                &quot;graph does not contain vertex: {0}&quot;.format(vertex)
            )

        # delete _adjacencies
        del self._adj[vertex]
        for adj in self._adj.values():
            adj[:] = [v for v in adj if v != vertex]

        # delete edges
        self.edges = [
            e for e in self.edges if vertex not in (e[0], e[1])
        ]

    def get_tails(self, head):
        &quot;&quot;&quot;
        Get list of vertices where a vertex is on the right side of an edge
        &quot;&quot;&quot;
        return [e[0] for e in self.edges if e[1] == head]

    def get_heads(self, tail):
        &quot;&quot;&quot;
        Get list of vertices where a vertex is on the left side of an edge
        &quot;&quot;&quot;
        return [e[1] for e in self.edges if e[0] == tail]

    def bfs(self, start=None):
        &quot;&quot;&quot;
        Breadth-first search traversal of the graph from `start` vertex.
        Return a set of all visited vertices
        &quot;&quot;&quot;
        if not start:
            start = next(iter(self.vertices))
        visited = set()
        queue = deque([start])

        while queue:
            vertex = queue.popleft()
            if vertex not in visited:
                visited.add(vertex)
                queue.extend(set(self._adj.get(vertex, [])) - visited)
        return visited
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