[List] append VS. extend

s = list(range(3))
s.append(10)
print(s)

s = list(range(3))
s.append([10])
print(s)

s = list(range(3))
s.extend([10])
print(s)

output

[0, 1, 2, 10]
[0, 1, 2, [10]]
[0, 1, 2, 10]