Weird MemoryError issue

[John Machin]

On 20/07/2006 1:58 AM, jedi200581 at yahoo.co.uk wrote:
> 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:

Syntax error. Should be:
    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))

Syntax error (missing :)

>     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 ??

Neither of the above compiles without error. You may regard this as a 
novel suggestion (it's not), but try pasting the actual code that you ran.

When corrected, the versions are functionally equivalent. They give the 
  same result: on my box, they fell over trying to do 
is_prime(536870911), in particular trying to do range(536870911). If 
successful, that would produce a list of approx 0.5G elements. On a 
32-bit box, that's about 2GB of pointers to objects. Each object has a 
reference count, a pointer to its type, and its value -- another 6GB. 
Total about 8GB. Use xrange() to avoid the memory grab.

HTH,
John