The most straightforward method would be to apply the formula
directly.
Loop on j computing Fj along the way
if n<=1 : return n
Fold=0
Fnew=1
for j in range(2,n) :
Fold, Fnew = Fnew, Fold+Fnew
return Fnew
Even simpler:
return round(((1+sqrt(5.))/2)**n/sqrt(5.))