sieve2.py

def Sieve(num):
    prime = [True for i in range(num+1)]
# boolean array
    p = 2
    while (p * p <= num):
 
        # If prime[p] is not
        # changed, then it is a prime
        if (prime[p] == True):
 
            # Updating all multiples of p
            for i in range(p * p, num+1, p):
                prime[i] = False
        p += 1
 
    # Print all prime numbers
    primes = open("primes.txt", "w")
    for p in range(2, num+1):
        if prime[p]:
            primes.write(str(p) + "\n")            
 
 
# Driver code
if __name__ == '__main__':
    num = 2000000
    print("Following are the prime numbers smaller"),
    print("than or equal to", num)
    Sieve(num)