input: Write a function to count the total number of vowels in a given string. If the string is empty, return 0. The function should be case-insensitive.
  def count_vowels(string): 
      vowels = "aeiou"
      count = 0
      for s in string:
          if s.lower() in vowels:
              count += 1
      return count
  
  assert count_vowels("Hello, World!")==3
  assert count_vowels("Python Programming")==4
  assert count_vowels("aeiouAEIOU")==10
  assert count_vowels("")==0
  assert count_vowels("Thx 4 n f")==1
output: def count_vowels(string): 
      vowels = "aeiou"
      count = 0
      for s in string:
          if s.lower() in vowels:
              count += 1
      return count