Weird MemoryError issue

[jedi200581 at yahoo.co.uk]

Hi,

  I'm new at python as I just started to learn it, but I found out
something weird. I have wrote a little program to compute Mersenne
number:

# Snipet on
def is_prime n:
  for i in range(2, n):
    if (n % i) == 0:
      return 0
  else:
      return 1

for a in range(2, 10000000):
  if (is_prime(a) and is_prime(2**a-1))
    print 2**a-1, " is a prime number"
# Snipet off

This program raise MemoryError. But this one:

# Snipet on
def is_prime n:
  for i in range(2, n):
    if (n % i) == 0:
      return 0
  return 1 # the change is here

for a in range(2, 10000000):
  if (is_prime(a) and is_prime(2**a-1))
    print 2**a-1, " is a prime number"
# Snipet off

Does not! Why ??

Thx.