How do you clone a graph?

from collections import deque

class Node:
    def __init__(self, val):
        self.val = val
        self.neighbors = []

def clone_graph(node):
    if not node:
        return None
    clones = {node: Node(node.val)}
    q = deque([node])
    while q:
        cur = q.popleft()
        for nb in cur.neighbors:
            if nb not in clones:
                clones[nb] = Node(nb.val)
                q.append(nb)
            clones[cur].neighbors.append(clones[nb])
    return clones[node]


# --- demo ---  square graph 1-2-3-4-1
a, b, c, d = Node(1), Node(2), Node(3), Node(4)
a.neighbors = [b, d]; b.neighbors = [a, c]
c.neighbors = [b, d]; d.neighbors = [a, c]
copy = clone_graph(a)
print(copy is not a)                              # True  (deep copy)
print(copy.val, [n.val for n in copy.neighbors])  # 1 [2, 4]