Dijkstra shortest path algorithm based on python heapq heap implementation

dijkstra.py

from collections import defaultdict
from heapq import *

def dijkstra(edges, f, t):
    g = defaultdict(list)
    for l,r,c in edges:
        g[l].append((c,r))

    q, seen, mins = [(0,f,())], set(), {f: 0}
    while q:
        (cost,v1,path) = heappop(q)
        if v1 not in seen:
            seen.add(v1)
            path = (v1, path)
            if v1 == t: return (cost, path)

            for c, v2 in g.get(v1, ()):
                if v2 in seen: continue
                prev = mins.get(v2, None)
                next = cost + c
                if prev is None or next < prev:
                    mins[v2] = next
                    heappush(q, (next, v2, path))

    return float("inf"), None

if __name__ == "__main__":
    edges = [
        ("A", "B", 7),
        ("A", "D", 5),
        ("B", "C", 8),
        ("B", "D", 9),
        ("B", "E", 7),
        ("C", "E", 5),
        ("D", "E", 15),
        ("D", "F", 6),
        ("E", "F", 8),
        ("E", "G", 9),
        ("F", "G", 11)
    ]

    print "=== Dijkstra ==="
    print edges
    print "A -> E:"
    print dijkstra(edges, "A", "E")
    print "F -> G:"
    print dijkstra(edges, "F", "G")

Thank you so much for this gift, very clean and clever solution 😄

If anyone just wonders how to easily receive as output only the value of the solution remove the cost from the return at line 15:

if v1 == t: return cost
instead of
if v1 == t: return (cost, path)

Nice and clean

Thank you very much for this beautiful algorithm.

pretty sure this is not Dijkstra; you're doing heappush(q, (next, v2, path)) at the very end, but in True dijkstra it would need a call to "decrease_key", which in python is heap._siftdown

@xdavidliu I was confused by this until I saw https://stackoverflow.com/a/31123108. I think Dijkstra's algorithm is a higher level concept, so either implementation is valid.

More concise with path reconstruction. The node IDs are represented as integers while the edge weights as floats

from typing import *
from heapq import *


class Dijkstra:
    def __init__(self,
                 graph: Dict[int, Dict[int, float]],
                 origin: int):
        self.graph = graph
        self.edge_to: Dict[int, int] = {}
        self.distances: Dict[int, float] = {vertex: float('inf') for vertex in self.graph}
        self.origin = origin
        self._find(self.origin)

    def _find(self, node: int):

        self.distances[node] = 0

        # Priority queue which stores tuples from distance to node id
        # Distance is the first in the tuple order since it needs to have
        # priority when entries are inserted into the priority queue
        priority_queue: List[(float, int)] = [(node, 0)]

        while priority_queue:
            current_node, current_distance = heappop(priority_queue)

            # If the distance currently recorded at the distances dict is
            # bigger than the one pushed to the pq then we do not need to
            # process this entry
            if current_distance > self.distances[current_node]:
                continue

            for n, weight in self.graph[current_node].items():
                updated_distance = current_distance + weight

                if updated_distance < self.distances[n]:
                    self.distances[n] = updated_distance
                    self.edge_to[n] = current_node
                    heappush(priority_queue, (n, updated_distance))

    def reconstruct_path(self, destination: int) -> List[int]:
        node: int = destination
        path: List[int] = []

        while node != self.origin:
            path.append(node)
            node = self.edge_to[node]

        path.reverse()
        return path


if __name__ == '__main__':
    graph = {
        1: {2: 1, 3: 4},
        2: {1: 1, 3: 2, 4: 5},
        3: {1: 4, 2: 2, 4: 1},
        4: {2: 5, 3: 1}
    }

    dijkstra: Dijkstra = Dijkstra(graph, 1)

    print(dijkstra.distances)
    print(dijkstra.reconstruct_path(4))
    print(dijkstra.reconstruct_path(2))
    print(dijkstra.reconstruct_path(3))

friends don't let friends import * 😄

to quote pep8

Wildcard imports (from module import *) should be avoided, as they make it unclear which names are present in the namespace, confusing both readers and many automated tools. There is one defensible use case for a wildcard import, which is to republish an internal interface as part of a public API (for example, overwriting a pure Python implementation of an interface with the definitions from an optional accelerator module and exactly which definitions will be overwritten isn’t known in advance).

When republishing names this way, the guidelines below regarding public and internal interfaces still apply.