Sieve of Eratosthenes

#Python code to find all primes in given range using Sieve of Eratosthenes algo.

def Sieve_of_Eratosthenes(n):
    composites=[]
    primes=[]
    for a in range(2, n+1):
        if a not in composites:
            primes.append(a)
        for b in range(a*a, n+1,a):
            composites.append(b)
    return primes       
print Sieve_of_Eratosthenes(1000)