Left truncatable primes

recurse.py

import math

def pt(x):
    x = int(x)
    for i in range(2, math.floor(math.sqrt(x))):
        if x % i == 0:
            return False
    return True
    
def checker(left=None, res=None):
    l = {"1", "3", "7", "9"}
    v = ["2", "3", "5", "7"]
    left = left or v
    res = res or set(v)
    for x in left:
        for i in l:
            if pt(x + i):
                res.add(x + i)
                checker([x + i], res)
    return sorted(list(map(int, res)))
        
    
checker()