def flatten(data):
a = []
for item in data:
if type(item) is list:
a += flatten(item)
else:
a.append(item)
print(a)
c=[[1,2,3],[4,[5,6]],7,[8,9]]
flatten(c)

'Nonetype' object is nor itrerable 라 뜨는데 뭐가 문제지