In [51]: stage = [[False]*3]*3

In [52]: stage
Out[52]: [[False, False, False], [False, False, False], [False, False, False]]

In [53]: stage[1][1] = True

In [54]: stage
Out[54]: [[False, True, False], [False, True, False], [False, True, False]]
#[0][1], [2][1]もTrueになっている