Python Dijkstra shortest paths

import heapq

def shortest_paths(graph, start):
    distances = {start: 0}
    heap = [(0, start)]
    while heap:
        distance, node = heapq.heappop(heap)
        if distance != distances[node]:
            continue
        for neighbor, weight in graph.get(node, []):
            if weight < 0:
                raise ValueError("nonnegative weights required")
            candidate = distance + weight
            if candidate < distances.get(neighbor, float("inf")):
                distances[neighbor] = candidate
                heapq.heappush(heap, (candidate, neighbor))
    return distances

graph = {"A": [("B", 4), ("C", 1)], "C": [("B", 2)], "B": []}
result = shortest_paths(graph, "A")
print(sorted(result.items()))
assert shortest_paths({}, "A") == {"A": 0}