下面的代码：

import sys
MOD = 10 ** 9
sys.setrecursionlimit(3000000)
primes = [2,3,5,7,11,13]
F = [1, 2]

cols = 1 + 2
rows = F[-1] + 2
memo = [[-1] * (cols)] * rows

def b(n, i):
    try:
        print("Called b(", n, i, "), whose memorised value currently =", memo[n][i])
        if n < 0:
            return 0
        assert(memo[2][0]<0)
        if memo[n][i] >= 0:
            print(n, i, "is alredy set")
            return memo[n][i]
        elif n == 0:
            print("setting memo(", n, i, ") to 1")
            memo[n][i] = 1
        elif i == 0:
            print("setting memo(", n, i, ") to 0")
            memo[n][i] = 0
        else:
            memo[n][i] = (b(n, i - 1) + b(n - primes[i-1], i) * primes[i-1]) % MOD
            print("setting recursively memo(", n, i, ") to ", memo[n][i])
        print("Returning from b(", n, i, "), whose memorised value after update =", memo[n][i])
        return memo[n][i]
    except Exception as e:
        print(e)
        raise e

print("memo=",memo)
n = 1
an = b(n, 0)
print("memo[2][0]=",memo[2][0])

作为调试工作，我在最后一行打印memo[2][0]。memo[2][0]应该是-1，因为它没有被递归更新，只是最初设置为-1。

memo= [[-1, -1, -1], [-1, -1, -1], [-1, -1, -1], [-1, -1, -1]]
Called b( 1 0 ), whose memorised value currently = -1
setting memo( 1 0 ) to 0
Returning from b( 1 0 ), whose memorised value after update = 0
memo[2][0]= 0

预期输出应为：

memo= [[-1, -1, -1], [-1, -1, -1], [-1, -1, -1], [-1, -1, -1]]
Called b( 1 0 ), whose memorised value currently = -1
setting memo( 1 0 ) to 0
Returning from b( 1 0 ), whose memorised value after update = 0
memo[2][0]= -1