input: 
  from typing import List
  
  
  def count_vowels(string): 
      vowels = "aeiou"
      count = 0
      for s in string:
          if s.lower() in vowels:
              count += 1
      return count
  
  # Test cases
  assert count_vowels("python") == 1
  assert count_vowels("hello world") == 3
  assert count_vowels("AEIOU") == 5
  assert count_vowels("") == 0
  
output:     vowels = "aeiou"
      count = 0
      for s in string:
          if s.lower() in vowels:
              count += 1
      return count