mytest=[[False]*3]*2

In [46]: mytest
Out[46]: [[False, False, False], [False, False, False]]

In [47]: mytest[0][1]=True

In [48]: mytest
Out[48]: [[False, True, False], [False, True, False]]

On the other hand

mytest=[ [False]*3 for i in range(2)]
In [53]: mytest[0][1]=True

In [54]: mytest
Out[54]: [[False, True, False], [False, False, False]]