Python 函数中的 return 失效了是什么鬼

# coding:utf-8
import numpy as np


def drop_water(y, x, array, path):
    s1 = array[y + 1, x - 1] * 5  # 左下
    s2 = array[y + 1, x] * 4  # 下
    s3 = array[y + 1, x + 1] * 3  # 右下
    s4 = array[y, x + 1] * 2  # 右
    s5 = array[y, x - 1] * 1  # 左
    l = [s1, s2, s3, s4, s5]
    w = 4 if sum(l) in [0, 15] else max(l)
    if w == 1:
        next_s = [y, x - 1]
    elif w == 2:
        next_s = [y, x + 1]
    elif w == 3:
        next_s = [y + 1, x + 1]
    elif w == 4:
        next_s = [y + 1, x]
    elif w == 5:
        next_s = [y + 1, x - 1]
    path.append(next_s)
    if next_s[0] == array.shape[0] - 1:
        print(path)
        return path
    else:
        print('a')
        drop_water(next_s[0], next_s[1], array, path)
        print('w')


array = np.array([[1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1,
                   1, 1, 1, 1, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 1,
                   1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1],
                  [1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 0, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0,
                   1, 1, 1, 1, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 1,
                   1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1],
                  [1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0,
                   0, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0,
                   1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1],
                  [1, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 0,
                   0, 0, 0, 1, 1, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0,
                   1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1]])

s = drop_water(0, 30, array, [])
print(s)

输出如下:

a
a
[[1, 30], [2, 30], [3, 29]]
w
w
None