Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import collections
import heapq


AdjacentVertex = collections.namedtuple("AdjacentVertex", "vertex cost")
INF = 2 ** 31 - 1
NO_VERTEX = -1


# Prim?????§??????0?????????????°???¨?????¨????±???????
def compute_mst_prim(max_v, adj_list):
    # pred[u]?????????u???????????¨???????????????????????¨???
    pred = collections.defaultdict(lambda: NO_VERTEX)
    # u??¨pred[u]????????¶???????????????key[u]?????\???
    key = collections.defaultdict(lambda: INF)
    key[0] = 0
    # ??????????????????????????????????????\?????¨????????¨??????
    pq = [(key[v], v) for v in range(max_v)]
    heapq.heapify(pq)
    while pq:
        _, u = heapq.heappop(pq)
        for v, v_cost in adj_list[u]:
            if v in (item[1] for item in pq):
                weight = v_cost
                if weight < key[v]:
                    pred[v] = u
                    key[v] = weight
                    heapq.heappush(pq, (weight, v))
    return (pred, key)


def main():
    max_v, max_e = map(int, input().split())
    adjacency_list = collections.defaultdict(set)
    for _ in range(max_e):
        s, t, w = map(int, input().split())
        adjacency_list[s].add(AdjacentVertex(t, w))
        adjacency_list[t].add(AdjacentVertex(s, w))
    (_, key) = compute_mst_prim(max_v, adjacency_list)
    print(sum(key.values()))


if __name__ == '__main__':
    main()

Fixed submission:
#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import array
import collections
import heapq


AdjacentVertex = collections.namedtuple("AdjacentVertex", "vertex cost")
INF = 2 ** 31 - 1
NO_VERTEX = -1


# Prim?????§??????0?????????????°???¨?????¨????±???????
def compute_mst_prim(max_v, adj_list):
    # pred[u]?????????u???????????¨???????????????????????¨???
    pred = collections.defaultdict(lambda: NO_VERTEX)
    # u??¨pred[u]????????¶???????????????key[u]?????\???
    key = collections.defaultdict(lambda: INF)
    key[0] = 0
    # ??????????????????????????????????????\?????¨????????¨??????
    pq = [(key[v], v) for v in range(max_v)]
    heapq.heapify(pq)
    # ????????????????????\?????????????????\??£???????????????????????????
    in_pq = array.array("B", (True for _ in range(max_v)))
    while pq:
        _, u = heapq.heappop(pq)
        in_pq[u] = False
        for v, v_cost in adj_list[u]:
            if in_pq[v]:
                weight = v_cost
                if weight < key[v]:
                    pred[v] = u
                    key[v] = weight
                    heapq.heappush(pq, (weight, v))
                    in_pq[v] = True
    return (pred, key)


def main():
    max_v, max_e = map(int, input().split())
    adjacency_list = collections.defaultdict(set)
    for _ in range(max_e):
        s, t, w = map(int, input().split())
        adjacency_list[s].add(AdjacentVertex(t, w))
        adjacency_list[t].add(AdjacentVertex(s, w))
    (_, key) = compute_mst_prim(max_v, adjacency_list)
    print(sum(key.values()))


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import collections
import heapq


AdjacentVertex = collections.namedtuple("AdjacentVertex", "vertex cost")
INF = 2 ** 31 - 1
NO_VERTEX = -1


# Prim?????§??????0?????????????°???¨?????¨????±???????
def compute_mst_prim(max_v, adj_list):
    # pred[u]?????????u???????????¨???????????????????????¨???
    pred = collections.defaultdict(lambda: NO_VERTEX)
    # u??¨pred[u]????????¶???????????????key[u]?????\???
    key = collections.defaultdict(lambda: INF)
    key[0] = 0
    # ??????????????????????????????????????\?????¨????????¨??????
    pq = []
    for v in range(max_v):
        heapq.heappush(pq, (key[v], v))
    while pq:
        _, u = heapq.heappop(pq)
        for v, v_cost in adj_list[u]:
            if v in (item[1] for item in pq):
                weight = v_cost
                if weight < key[v]:
                    pred[v] = u
                    key[v] = weight
                    heapq.heappush(pq, (weight, v))
    return (pred, key)


def main():
    max_v, max_e = map(int, input().split())
    adjacency_list = collections.defaultdict(set)
    for _ in range(max_e):
        s, t, w = map(int, input().split())
        adjacency_list[s].add(AdjacentVertex(t, w))
        adjacency_list[t].add(AdjacentVertex(s, w))
    #(_, key) = compute_mst_prim(max_v, adjacency_list)
    print(sum(key.values()))


if __name__ == '__main__':
    main()

Fixed submission:
#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import array
import collections
import heapq


AdjacentVertex = collections.namedtuple("AdjacentVertex", "vertex cost")
INF = 2 ** 31 - 1
NO_VERTEX = -1


# Prim?????§??????0?????????????°???¨?????¨????±???????
def compute_mst_prim(max_v, adj_list):
    # pred[u]?????????u???????????¨???????????????????????¨???
    pred = collections.defaultdict(lambda: NO_VERTEX)
    # u??¨pred[u]????????¶???????????????key[u]?????\???
    key = collections.defaultdict(lambda: INF)
    key[0] = 0
    # ??????????????????????????????????????\?????¨????????¨??????
    pq = [(key[v], v) for v in range(max_v)]
    heapq.heapify(pq)
    # ????????????????????\?????????????????\??£???????????????????????????
    in_pq = array.array("B", (True for _ in range(max_v)))
    while pq:
        _, u = heapq.heappop(pq)
        in_pq[u] = False
        for v, v_cost in adj_list[u]:
            if in_pq[v]:
                weight = v_cost
                if weight < key[v]:
                    pred[v] = u
                    key[v] = weight
                    heapq.heappush(pq, (weight, v))
                    in_pq[v] = True
    return (pred, key)


def main():
    max_v, max_e = map(int, input().split())
    adjacency_list = collections.defaultdict(set)
    for _ in range(max_e):
        s, t, w = map(int, input().split())
        adjacency_list[s].add(AdjacentVertex(t, w))
        adjacency_list[t].add(AdjacentVertex(s, w))
    (_, key) = compute_mst_prim(max_v, adjacency_list)
    print(sum(key.values()))


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import collections
import heapq


AdjacentVertex = collections.namedtuple("AdjacentVertex", "vertex cost")
INF = 2 ** 31 - 1
NO_VERTEX = -1


# Prim?????§??????0?????????????°???¨?????¨????±???????
def compute_mst_prim(max_v, adj_list):
    # pred[u]?????????u???????????¨???????????????????????¨???
    pred = collections.defaultdict(lambda: NO_VERTEX)
    # u??¨pred[u]????????¶???????????????key[u]?????\???
    key = collections.defaultdict(lambda: INF)
    key[0] = 0
    # ??????????????????????????????????????\?????¨????????¨??????
    pq = []
    for v in range(max_v):
        heapq.heappush(pq, (key[v], v))
    while pq:
        _, u = heapq.heappop(pq)
        for v, v_cost in adj_list[u]:
            if v in (item[1] for item in pq):
                weight = v_cost
                if weight < key[v]:
                    pred[v] = u
                    key[v] = weight
                    heapq.heappush(pq, (weight, v))
    return (pred, key)


def main():
    max_v, max_e = map(int, input().split())
    adjacency_list = collections.defaultdict(set)
    for _ in range(max_e):
        s, t, w = map(int, input().split())
        adjacency_list[s].add(AdjacentVertex(t, w))
        adjacency_list[t].add(AdjacentVertex(s, w))
    (_, key) = compute_mst_prim(max_v, adjacency_list)
    print(sum(key.values()))


if __name__ == '__main__':
    main()

Fixed submission:
#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import array
import collections
import heapq


AdjacentVertex = collections.namedtuple("AdjacentVertex", "vertex cost")
INF = 2 ** 31 - 1
NO_VERTEX = -1


# Prim?????§??????0?????????????°???¨?????¨????±???????
def compute_mst_prim(max_v, adj_list):
    # pred[u]?????????u???????????¨???????????????????????¨???
    pred = collections.defaultdict(lambda: NO_VERTEX)
    # u??¨pred[u]????????¶???????????????key[u]?????\???
    key = collections.defaultdict(lambda: INF)
    key[0] = 0
    # ??????????????????????????????????????\?????¨????????¨??????
    pq = [(key[v], v) for v in range(max_v)]
    heapq.heapify(pq)
    # ????????????????????\?????????????????\??£???????????????????????????
    in_pq = array.array("B", (True for _ in range(max_v)))
    while pq:
        _, u = heapq.heappop(pq)
        in_pq[u] = False
        for v, v_cost in adj_list[u]:
            if in_pq[v]:
                weight = v_cost
                if weight < key[v]:
                    pred[v] = u
                    key[v] = weight
                    heapq.heappush(pq, (weight, v))
                    in_pq[v] = True
    return (pred, key)


def main():
    max_v, max_e = map(int, input().split())
    adjacency_list = collections.defaultdict(set)
    for _ in range(max_e):
        s, t, w = map(int, input().split())
        adjacency_list[s].add(AdjacentVertex(t, w))
        adjacency_list[t].add(AdjacentVertex(s, w))
    (_, key) = compute_mst_prim(max_v, adjacency_list)
    print(sum(key.values()))


if __name__ == '__main__':
    main()