input: Write a python function to return a list of prime numbers up to a given number.
  assert print_primes(10)==[2, 3, 5, 7]
  assert print_primes(20)==[2, 3, 5, 7, 11, 13, 17, 19]
  assert print_primes(5)==[2, 3, 5]
output: def print_primes(n):
      def is_prime(x):
          if x < 2:
              return False
          for i in range(2, int(x**0.5) + 1):
              if x % i == 0:
                  return False
          return True
  
      primes = [x for x in range(2, n+1) if is_prime(x)]
      return primes