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