import itertools
import math
 
 
def is_prime(n):
    assert n >= 1, f"n must be >= 1, is {n}"
    if n == 1:
        return False
    if n % 2 == 0:
        return True
 
    for i in range(3, math.floor(math.sqrt(n))+1, 2):
        if n % i == 0:
            return False
 
    return True
 
 
only_pair_prime = iter([2])
odd_numbers = itertools.count(3, 2)
prime_candidates = itertools.chain(only_pair_prime, odd_numbers)
primes = filter(is_prime, prime_candidates)
first_20 = itertools.islice(primes, 20)
print(list(first_20))
 

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