Facing MemoryError in Python

Question

I wrote a program to find prime factors of a number. When I give a large number(600851475143) as input, MemoryError pops up. Below is the code:

def fact(a):
    factors = []
    for i in range(1,a+1):
        if a%i == 0:
            factors.append(i)
    return factors 

num = raw_input(">>  ")
#600851475143
a = b = []
a = fact(long(num))    
for i in range(0,len(a)):
    b = fact(a[i])
    if len(b) <= 2:
        print a[i]

From browsing I came to know that Python makes use of Computer memory - RAM. I am using Python in Unbuntu without changing its configuration. Should I change anythig to work on 64-bit machine. Or should I use any additional function(s) to get around this error

Answer 1

There are various ways to measure the memory used by your program, and you may be able increase per-user limits or something.

However, you don't need to allocate that memory in the first place, since you can just generate the sequence without storing it all:

def fact(a):
    "just return a generator for the sequence you want"
    return (i for i in xrange(1,a+1) if a % i == 0)

Note that you can also iterate directly over sequences without needing to index them repeatedly:

for b in fact(long(num)):
    print b