python list[:] operation

I came across a question about the difference between alist[:] = blist and alist = blist[:], and surprisingly found no one has mentioned the id difference of these two operations.

To know why I have this kind of curiosity: python-site-module, you will find that:

...
    sys.path[:] = L
    return known_paths

I know this function is used to filter out repeats inside sys.path, but why does it have this kind of assignment? How about sys.path = L, sys.path = L[:] and etc..?

Digging deeper, I then know the fundamental idea behind this operation, they want to keep constant of the ID memory address.