Python 3.8 :=

[perfringo]

So it looks like y := x would fail by itself.

Yep, it is so:

>>> x := 5
  File "<stdin>", line 1
    x := 5
      ^
SyntaxError: invalid syntax
>>> (x := 5)
5

I prefer to use assignment expressionn in cases where it actually shortens the code, avoids calculation of same value and makes it more concise.

Example of calculating once (minimum number of page flips to location starting either from beginning or end):

min(from_start := location // 2, total_pages // 2 - from_start)