function giving different result with @functool.cache

[naughtysensei]

        prerow = [1].append(prerow)  # Makes a new list.  Fixes shared list problem.
        for i in range(1, len(prerow) - 1):
            prerow[i] += prerow[i+1]
        return prerow

you mean this:

one = [1]
one.extend(prerow)
prerow = one
for i in range(1, len(prerow)-1):
    prerow[i] += prerow[i+1]
return prerow

or even better I guess:

prerow = prerow.copy()
for i in range(len(prerow)-1):
    prerow[i] += prerow[i+1]
return [1] + prerow

It is now obvious where I fucked up. thx. "Make a new list" this helped.