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Check only up to √n

n is prime if nothing from 2 up to √n divides it: if n = a × b, one of them is at most √n. Loop while i * i <= n.

1def is_prime(n):
2    if n < 2:
3        return False
4    i = 2
5    while i * i <= n:
6        if n % i == 0:
7            return False
8        i += 1
9    return True