Prime tree elements

primes.py

primes = [2, 3, 5, 7]
parent = [-1, -1, -1, -1]
digits = [1, 3, 7, 9]

def isprime(N):
    i = 2
    while i * i <= N:
        if N % i == 0:
            return False
        i += 1
    return True

i = 0
while i < len(primes):
    p = primes[i]
    for d in digits:
        if isprime(p * 10 + d):
            primes.append(p * 10 + d)
            parent.append(i)
    i += 1

print(len(primes))
print(primes)