input: Write a function to count the total number of vowels in a given string. Vowels are defined as 'a', 'e', 'i', 'o', 'u' and both uppercase and lowercase should be considered. If the input string is empty, return 0.
  
  def count_vowels(string): 
      vowels = "aeiouAEIOU"
      count = 0
      for s in string:
          if s in vowels:
              count += 1
      return count
  
  assert count_vowels("Hello World")==3
  assert count_vowels("AEIOU")==5
  assert count_vowels("Python Programming")==4
  assert count_vowels("")==0
output: def count_vowels(string): 
      vowels = "aeiouAEIOU"
      count = 0
      for s in string:
          if s in vowels:
              count += 1
      return count