Initialize parameter in recursive function in Python

Question

I wrote down simple algorithm for checking if Mersenne number is prime:

def is_prime_mers(result, step, modulo):
    if result != 0:
        if step == 0:
            return False
        result = result ** 2 - 2
        if result >= modulo:
            result %= modulo
        return is_prime_mers(result, step - 1, modulo)
    return True

Generally I don't need to give result parameter when script calls this function however I need it for recursive calls.

So only result need to be initialised for this function with value of 4

I can write some initializing function like:

def is_prime_mers_init(step):
    is_prime_mers(4, step, count_mersenne(step))

But maybe there is some python/general programming pattern to do that in first function?

Edit: For all curious ones :) This is function what implements Lucas-Lehmer test and checks if given Mersenne number is prime http://en.wikipedia.org/wiki/Lucas%E2%80%93Lehmer_primality_test - however I read only mathematic implementation - so this is purely my code solution.

step is number of recursive calls

count_mersenne(step) gives value of some Mersenne number: 2 ** step - 1 however I use some check in count_mersenne(step) because searching for prime Mersenne numbers can be limited to prime step values.

Answer 1

You can assign them dummy default values and then you can decide whether to change them or not, like this

def is_prime_mers(step, result = None, modulo = None):
    if result is None:
        result = 4                    # Default value
    if modulo is None:
        modulo = count_mersenne(step)  # Default value

Or in one liners

def is_prime_mers(step, result = None, modulo = None):
    result = 4 if result is None else result
    modulo = count_mersenne(step) if modulo is None else modulo