how good is the optimization?

[nilamo]

how good is the optimization in Python? in this code will it cache the value from s[n] or will it fetch it twice?

   if s[n]<128 or s[n]>191:
        return -2

or should i code this?

    x = s[n]
    if x<128 or x>191:
        return -2

for best performance.

I'm not sure this is an optimization issue. s[n] could have a different value every time it's accessed, so it'd HAVE to be accessed twice. This is easy enough to test, even if you don't want to dig into the dis module:

>>> class foo:
...   def __getitem__(self, index):
...     print("indexed!")
...     return index ** 2
...
>>> x = foo()
>>> x[4]
indexed!
16
>>> if x[4] > 10 and x[4] < 50:
...   print("match")
...
indexed!
indexed!
match