Activating a View node to update contents of 'Viewer Node' image data block [Python]

NOTE: It might be helpful if you take a look at the main post here on blender.stackexchange.com since some people have posted answers (which are not useful for me as now now) and it might help you better understand what I want to do .

I am using two Viewer nodes to directly access pixel values of the rendering through

bpy.data.images['Viewer Node'].pixels

I want to access the outputs of both of these nodes by activating them one by one using Python. However, it seems that I cannot activate them via Python. If I click on either of them the Viewer Node data block is immediately updated as the sum of the pixel values shows. For instance, clicking on Viewer2:

https://i.stack.imgur.com/QRkt7.png

And doing

np.array(bpy.data.images['Viewer Node'].pixels[:]).sum()

will give me 292007.42460217333

Clicking on Viewer1 and doing

np.array(bpy.data.images['Viewer Node'].pixels[:]).sum()

will result in a different number however. Doing the followings will not activate either of the Viewer nodes and I will always get the same number in the np.sum():

v1 = bpy.context.scene.node_tree.nodes['Viewer']
v2 = bpy.context.scene.node_tree.nodes['Viewer.001']

v1.select = True
v2.select = False

np.array(bpy.data.images['Viewer Node'].pixels[:]).sum()
292007.42460217333`

v1.select = False
v2.select = True

np.array(bpy.data.images['Viewer Node'].pixels[:]).sum()
292007.42460217333

I also tried

bpy.context.scene.node_tree.nodes.active = v1

after

v1.select = True

and

v2.select = False

lines but it still does not work.

None of these activate either of the Viewer nodes. It seems that I am doing the same thing as shown in the solutions provided here but I cannot activate the nodes, or update the image block without re-rendering. Another solution that I found is this one. However, it seems that Blender’s Python API has changed a little bit for the newer versions of Blender (I am using 2.79) and I cannot use the method provided.
A promising solution seems to be coming from invoking clicking events as shown here but I don’t know how I can do it for selecting nodes. So I wonder, does any one know how to activate the Viewer nodes via Python, possibly via invoking LBM events?