python中的 += 操作是 in-place，会覆盖原始内存

You avoid the in-place by doing

out = out + residual

You can verify that with other data structures as well:

a = []
b = a  # b is the same as a
a += [5]
print(b)  # prints [5]

# now
a = []
b = a  # b is the same as a
a = a + [5]
print(b)  # prints []