Showing exponential time complexity of the simple recursive method

fibo.py

import timeit
def fib(n):
    if n==1 or n==2:
        return 1
    return fib(n-1) + fib(n-2)

print(timeit.timeit("fib(10)", "from __main__ import fib", number=1, ))
print(timeit.timeit("fib(20)", "from __main__ import fib", number=1, ))
print(timeit.timeit("fib(30)", "from __main__ import fib", number=1, ))
print(timeit.timeit("fib(40)", "from __main__ import fib", number=1, ))