input: Write a function to identify and return the list of prime numbers from a given list of integers. The function should return an empty list if no prime numbers are present. If the input list is empty, the function should also return an empty list. 
  
  def is_prime(num):
   for i in range(2, num // 2 + 1):
   if num % i == 0:
   return False
   return num > 1
  
  def extract_primes(lst):
   prime_lst = []
   for num in lst:
   if is_prime(num):
   prime_lst.append(num)
   return prime_lst
  
  assert extract_primes([2, 8, 3, 6, 10, 7, 14, 11, 17, 15]) == [2, 3, 7, 11, 17]
  assert extract_primes([4, 6, 8, 9, 10, 12, 14, 15]) == []
  assert extract_primes([]) == []
output: def extract_primes(lst):
      prime_lst = []
      for num in lst:
          if is_prime(num):
              prime_lst.append(num)
      return prime_lst