参考:https://www.cnblogs.com/traditional/p/12422934.html
使用 yield
yield 返回的是一个迭代器，所以要用列表推导式将所有元素提取到列表中去。
def flatten(l: list)-> iter:
"""将列表扁平化"""
for _ in l:
if isinstance(_, list):
yield from flatten(_)
else:
yield _
>>> list1 = [1,2,3,[33,41,331,4,1,[1,2,3],3,[1]]]
>>> list2 = [_ for _ in flatten(list1)]
>>> list2
[1, 2, 3, 33, 41, 331, 4, 1, 1, 2, 3, 3, 1]