how good is the optimization?

[Gribouillis]

In this version, there are only two LOAD_FAST and no STORE_FAST

>>> def func3(s, n):
...     if not (128 <= s[n] <= 191):
...         return -2
... 
>>> import dis
>>> dis.dis(func3)
  2           0 LOAD_CONST               1 (128)
              3 LOAD_FAST                0 (s)
              6 LOAD_FAST                1 (n)
              9 BINARY_SUBSCR
             10 DUP_TOP
             11 ROT_THREE
             12 COMPARE_OP               1 (<=)
             15 JUMP_IF_FALSE_OR_POP    27
             18 LOAD_CONST               2 (191)
             21 COMPARE_OP               1 (<=)
             24 JUMP_FORWARD             2 (to 29)
        >>   27 ROT_TWO
             28 POP_TOP
        >>   29 POP_JUMP_IF_TRUE        36

  3          32 LOAD_CONST               4 (-2)
             35 RETURN_VALUE
        >>   36 LOAD_CONST               0 (None)
             39 RETURN_VALUE