def myflatten(slist, outlist):
for sl in slist:
if type(sl) == list:
outlist.append(myflatten(sl, outlist))
else:
outlist.append(sl)
return outlist
print("myflatten list=", myflatten([1,[5,6,7],3,4,[7,8,9]], []))
ZGVmIG15ZmxhdHRlbihzbGlzdCwgb3V0bGlzdCk6CiAgICBmb3Igc2wgaW4gc2xpc3Q6CiAgICAgICAgaWYgdHlwZShzbCkgPT0gbGlzdDogICAgCiAgICAgICAgICAgIG91dGxpc3QuYXBwZW5kKG15ZmxhdHRlbihzbCwgb3V0bGlzdCkpCiAgICAgICAgZWxzZTogCiAgICAgICAgICAgIG91dGxpc3QuYXBwZW5kKHNsKQogICAgcmV0dXJuIG91dGxpc3QKCnByaW50KCJteWZsYXR0ZW4gbGlzdD0iLCBteWZsYXR0ZW4oWzEsWzUsNiw3XSwzLDQsWzcsOCw5XV0sIFtdKSk=
('myflatten list=', [1, 5, 6, 7, [...], 3, 4, 7, 8, 9, [...]])