root = ['b']
def change(root):
    for index, item in enumerate(root):
        if isinstance(item, list):
            change(item)
        elif item == 'b':
            root[index] = ['w','w','w']
        elif item == 'w':
            root[index] = ['b','b']
    return root

for i in range(3):
    tree=change(root)
print(tree)