Colab 

x = [
    [1,2,3],
    [4,5,6]
]
y=x


y.append([7,8,9])


y


[[1, 2, 3], [4, 5, 6], [7, 8, 9]]

x


[[1, 2, 3], [4, 5, 6], [7, 8, 9]]

y = list(x)


y.append([10, 11, 12])


y


[[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]]

x


[[1, 2, 3], [4, 5, 6], [7, 8, 9]]

y[0].append('hello')


y


[[1, 2, 3, 'hello'], [4, 5, 6], [7, 8, 9], [10, 11, 12]]

x


[[1, 2, 3, 'hello'], [4, 5, 6], [7, 8, 9]]

import copy


y = copy.deepcopy(x)


x


[[1, 2, 3, 'hello'], [4, 5, 6], [7, 8, 9]]

y


[[1, 2, 3, 'hello'], [4, 5, 6], [7, 8, 9]]

y[0].append('goodbye')


y


[[1, 2, 3, 'hello', 'goodbye'], [4, 5, 6], [7, 8, 9]]

x


[[1, 2, 3, 'hello'], [4, 5, 6], [7, 8, 9]]