Dijkstra in Python with heapq

import heapq
from collections import defaultdict

def dijkstra(graph, source):
    """Shortest distance from source to every reachable node.

    graph: dict-of-lists where graph[u] = [(v, weight), ...]
    """
    dist = {source: 0}
    heap = [(0, source)]   # (distance, node)
    while heap:
        d, u = heapq.heappop(heap)
        if d > dist[u]:
            continue       # stale entry left over from a tighter relaxation
        for v, w in graph[u]:
            nd = d + w
            if nd < dist.get(v, float('inf')):
                dist[v] = nd
                heapq.heappush(heap, (nd, v))
    return dist

# Build a small weighted graph.
graph = defaultdict(list)
for u, v, w in [('A', 'B', 4), ('A', 'C', 1), ('C', 'B', 2),
                ('B', 'D', 1), ('C', 'D', 5), ('D', 'E', 3)]:
    graph[u].append((v, w))
    graph[v].append((u, w))   # undirected

print(dijkstra(graph, 'A'))
# {'A': 0, 'C': 1, 'B': 3, 'D': 4, 'E': 7}

# Disconnected: 'X' has no edges, only itself reachable.
lonely = defaultdict(list)
lonely['X'] = []
print(dijkstra(lonely, 'X'))   # {'X': 0}