The example in the code was not a side-effect. They were creating a new list by invoking a function on every member of the old list.

Here's the same thing re-written in Python:

  d = [compute_diffraction_at_wavelength(x, WAVELENGTH) for x in LensLayers]

Or oldschool Python (and still faster, I think):

  d = map(compute_diffraction_at_wavelength(WAVELENGTH), LensLayers)