Representing Graphs Using an Adjacency List in Python

from collections import defaultdict

class Graph:
    def __init__(self, directed=False):
        self._graph = defaultdict(list)
        self._directed = directed

    def add_edge(self, u, v):
        """Adds an edge from node u to node v."""
        self._graph[u].append(v)
        if not self._directed:
            self._graph[v].append(u) # For undirected graphs, add edge in reverse too

    def get_neighbors(self, node):
        """Returns a list of neighbors for a given node."""
        return self._graph[node]

    def get_nodes(self):
        """Returns a list of all nodes in the graph."""
        return list(self._graph.keys())

    def __str__(self):
        graph_str = "Graph adjacency list:
"
        for node in sorted(self._graph.keys()):
            graph_str += f"  {node}: {self._graph[node]}
"
        return graph_str

# Example Usage - Undirected Graph:
undirected_graph = Graph(directed=False)
undirected_graph.add_edge('A', 'B')
undirected_graph.add_edge('A', 'C')
undirected_graph.add_edge('B', 'D')
undirected_graph.add_edge('C', 'E')
undirected_graph.add_edge('D', 'E')
print("
--- Undirected Graph ---")
print(undirected_graph)
# Expected output:
# A: ['B', 'C']
# B: ['A', 'D']
# C: ['A', 'E']
# D: ['B', 'E']
# E: ['C', 'D']

# Example Usage - Directed Graph:
directed_graph = Graph(directed=True)
directed_graph.add_edge('X', 'Y')
directed_graph.add_edge('X', 'Z')
directed_graph.add_edge('Z', 'Y')
print("
--- Directed Graph ---")
print(directed_graph)
# Expected output:
# X: ['Y', 'Z']
# Z: ['Y']