Python 展开嵌套的list

将含有多层嵌套的list展开

isinstance()函数用法
isinstance(1, int)
//如果1是Int类型则返回True，否则返回False

// 
arr = [1, 3, [5, 6, [9, 10], [11, [12, [13, 14]]], 15]]

def fit(arr):
    return sum(([x] if not isinstance(x, list) else fit(x) for x in arr), [])

print(fit(arr))
# 展开结果
[1, 3, 5, 6, 9, 10, 11, 12, 13, 14, 15]