gistfile1.py

let square x = x *. x;;
square 10;;

def square(x): x**x
square(10)

let rec fib n = if n < 2 then 1 else fib(n-1) + fib(n-2);;
fib 10;;

rec fib(n):
    if n < 2:
        1
    else:
        fib(n-1) + fib(n-2)

or even just

rec fib(n): if n <2 then 1 else fib(n-1) + fib(n-2)

fib(10)