How does Dijkstra's algorithm work?

import heapq

def dijkstra(graph, source):
    # graph: {node: [(neighbor, weight), ...]}
    dist = {source: 0}
    heap = [(0, source)]
    while heap:
        d, u = heapq.heappop(heap)
        if d > dist.get(u, float('inf')):
            continue                    # stale entry
        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


# --- demo ---  nodes A=0 B=1 C=2 D=3
graph = {0: [(1, 4), (2, 1)], 1: [(3, 1)], 2: [(1, 2), (3, 5)], 3: []}
print(dijkstra(graph, 0))   # {0: 0, 2: 1, 1: 3, 3: 4}