Python Sieve of Eratosthenes

sieve.py

def count_primes(num):
    list = [True for i in range(num+1)]
    p = 2
    while p*p <= num:
        if list[p] == True:
            for i in range(p*p, num+1, p):
                list[i] = False
        p += 1
    return list.count(True) - 2