Custom Split Normals in view vs. in python

Question

I have a mesh with custom split normals. When I look at the normals on the loops through python I see the values I expect, but when I look at the split normals visually in edit mode, they are quite different--and the shading matches what the view is showing.

How do I get the view to show the custom normal data I see in the mesh data with python?

I've tried C.object.data.calc_normals_split() but that copies the values I see in the view into the values I see in python. I want to go the other way.

Details:

C.object.data.loops[1967].vertex_index 
492

C.object.data.loops[1967].normal
Vector((-0.5192142128944397, 0.7101643681526184, 0.4754820466041565))

Viewing the object in edit mode with vertex 492 selected, showing split normals:

You can see the y and z values of this split normal are negative whereas they are positive when I printed the loop normal.

Answer 1

Turns out I needed to run update() on the mesh before trying to set the normals, and then I never got into the weird state in the question. I would love a tutorial on when the update() and calc_*() functions need to be called.