Buggy python submission (verdict: Time Limit Exceeded):
class SquareMatrix():
    def __init__(self, n, mod=1000000007):
        self.n = n
        self.mat = [[0 for j in range(n)] for i in range(n)]
        self.mod = mod

    @staticmethod
    def id(n, mod=1000000007):
        res = SquareMatrix(n, mod)
        for i in range(n):
            res.mat[i][i] = 1
        return res

    @staticmethod
    def modinv(n, mod):
        assert n % mod != 0
        c0, c1 = n, mod
        a0, a1 = 1, 0
        b0, b1 = 0, 1
        while c1:
            a0, a1 = a1, a0 - c0 // c1 * a1
            b0, b1 = b1, b0 - c0 // c1 * b1
            c0, c1 = c1, c0 % c1
        return a0 % mod

    def set(self, arr):
        for i in range(self.n):
            for j in range(self.n):
                self.mat[i][j] = arr[i][j] % self.mod

    def operate(self, vec):
        assert len(vec) == self.n
        res = [0 for _ in range(self.n)]
        for i in range(self.n):
            for j in range(self.n):
                res[i] += self.mat[i][j] * vec[j]
                res[i] %= self.mod
        return res

    def add(self, other):
        assert other.n == self.n
        res = SquareMatrix(self.n, self.mod)
        for i in range(self.n):
            for j in range(self.n):
                res.mat[i][j] = self.mat[i][j] + other.mat[i][j]
                res.mat[i][j] %= self.mod
        return res

    def subtract(self, other):
        assert other.n == self.n
        res = SquareMatrix(self.n, self.mod)
        for i in range(self.n):
            for j in range(self.n):
                res.mat[i][j] = self.mat[i][j] - other.mat[i][j]
                res.mat[i][j] %= self.mod
        return res

    def times(self, k):
        res = SquareMatrix(self.n, self.mod)
        for i in range(self.n):
            for j in range(self.n):
                res.mat[i][j] = self.mat[i][j] * k
                res.mat[i][j] %= self.mod
        return res

    def multiply(self, other):
        assert self.n == other.n
        res = SquareMatrix(self.n, self.mod)
        for i in range(self.n):
            for j in range(self.n):
                for k in range(self.n):
                    res.mat[i][j] += self.mat[i][k] * other.mat[k][j]
                    res.mat[i][j] %= self.mod
        return res

    def power(self, k):
        tmp = SquareMatrix(self.n, self.mod)
        for i in range(self.n):
            for j in range(self.n):
                tmp.mat[i][j] = self.mat[i][j]
        res = SquareMatrix.id(self.n, self.mod)
        while k:
            if k & 1:
                res = res.multiply(tmp)
            tmp = tmp.multiply(tmp)
            k >>= 1
        return res

    def trace(self):
        res = 0
        for i in range(self.n):
            res += self.mat[i][i]
            res %= self.mod
        return res

    def determinant(self):
        res = 1
        tmp = SquareMatrix(self.n, self.mod)
        for i in range(self.n):
            for j in range(self.n):
                tmp.mat[i][j] = self.mat[i][j]
        for j in range(self.n):
            if tmp.mat[j][j] == 0:
                for i in range(j + 1, self.n):
                    if tmp.mat[i][j] != 0:
                        idx = i
                        break
                else:
                    return 0
                for k in range(self.n):
                    tmp.mat[j][k], tmp.mat[idx][k] = tmp.mat[idx][k], tmp.mat[j][k]
                res *= -1
            inv = SquareMatrix.modinv(tmp.mat[j][j], self.mod)
            for i in range(j + 1, self.n):
                c = -inv * tmp.mat[i][j] % self.mod
                for k in range(self.n):
                    tmp.mat[i][k] += c * tmp.mat[j][k]
                    tmp.mat[i][k] %= self.mod
        for i in range(self.n):
            res *= tmp.mat[i][i]
            res %= self.mod
        return res

    def transpose(self):
        res = SquareMatrix(self.n, self.mod)
        for i in range(self.n):
            for j in range(self.n):
                res.mat[i][j] = self.mat[j][i]
        return res

    def inverse(self): #self.determinant() != 0
        res = SquareMatrix.id(self.n, self.mod)
        tmp = SquareMatrix(self.n, self.mod)
        sgn = 1
        for i in range(self.n):
            for j in range(self.n):
                tmp.mat[i][j] = self.mat[i][j]
        for j in range(self.n):
            if tmp.mat[j][j] == 0:
                for i in range(j + 1, self.n):
                    if tmp.mat[i][j] != 0:
                        idx = i
                        break
                else:
                    return 0
                for k in range(self.n):
                    tmp.mat[j][k], tmp.mat[idx][k] = tmp.mat[idx][k], tmp.mat[j][k]
                    res.mat[j][k], res.mat[idx][k] = res.mat[idx][k], res.mat[j][k]
            inv = SquareMatrix.modinv(tmp.mat[j][j], self.mod)
            for k in range(self.n):
                tmp.mat[j][k] *= inv
                tmp.mat[j][k] %= self.mod
                res.mat[j][k] *= inv
                res.mat[j][k] %= self.mod
            for i in range(self.n):
                c = tmp.mat[i][j]
                for k in range(self.n):
                    if i == j:
                        continue
                    tmp.mat[i][k] -= tmp.mat[j][k] * c
                    tmp.mat[i][k] %= self.mod
                    res.mat[i][k] -= res.mat[j][k] * c
                    res.mat[i][k] %= self.mod
        return res

    def linear_equations(self, vec): #self.determinant != 0
        return self.inverse().operate(vec)

    def print(self):
        print(*self.mat, sep='\n')

MOD = 1000000007

N, K = map(int, input().split())
A = [tuple(map(int, input().split())) for _ in range(N)]

M = SquareMatrix(N)
M.set(A)

P = M.power(K)

res = 0

for i in range(N):
    for j in range(N):
        res += P.mat[i][j]
        res %= MOD

print(res)

Fixed submission:
MOD = 10**9+7

N, K = map(int, input().split())
A = [tuple(map(int, input().split())) for _ in range(N)]

def power(base, exp, op=lambda x, y: x * y, ie=1):
    res = ie
    while exp:
        if exp & 1:
            res = op(res, base)
        base = op(base, base)
        exp >>= 1
    return res

matrix_id = [[i == j for j in range(N)] for i in range(N)]

def matrix_mul(A, B):
    res = [[0 for j in range(N)] for i in range(N)]
    for i in range(N):
        for j in range(N):
            for k in range(N):
                res[i][j] += A[i][k] * B[k][j]
                res[i][j] %= MOD
    return res

P = power(A, K, matrix_mul, matrix_id)

res = 0

for i in range(N):
    for j in range(N):
        res += P[i][j]
        res %= MOD

print(res)

Buggy python submission (verdict: Time Limit Exceeded):
MOD = 10**9+7

N, K = map(int, input().split())
A = [tuple(map(int, input().split())) for _ in range(N)]

def power(base, exp, op=lambda x, y: x * y, ie=1):
    res = ie
    while exp:
        if exp & 1:
            res = op(res, base)
        base = op(base, base)
        exp >>= 1
    return res

matrix_id = [[i == j for j in range(N)] for i in range(N)]

def matrix_mul(A, B):
    res = [[0 for j in range(N)] for i in range(N)]
    for i in range(N):
        for j in range(N):
            for k in range(N):
                res[i][j] += A[i][k] * B[k][j]
                res[i][j] %= MOD
    return res

P = power(A, K, matrix_mul, matrix_id)

res = 0

for i in range(N):
    for j in range(N):
        res += P[i][j]
        res %= MOD

print(res)

Fixed submission:
MOD = 10**9+7

N, K = map(int, input().split())
A = [tuple(map(int, input().split())) for _ in range(N)]

def power(base, exp, op=lambda x, y: x * y, ie=1):
    res = ie
    while exp:
        if exp & 1:
            res = op(res, base)
        base = op(base, base)
        exp >>= 1
    return res

matrix_id = [[i == j for j in range(N)] for i in range(N)]

def matrix_mul(A, B):
    res = [[0 for j in range(N)] for i in range(N)]
    for i in range(N):
        for j in range(N):
            for k in range(N):
                res[i][j] += A[i][k] * B[k][j]
                res[i][j] %= MOD
    return res

P = power(A, K, matrix_mul, matrix_id)

res = 0

for i in range(N):
    for j in range(N):
        res += P[i][j]
        res %= MOD

print(res)