input: Write a function in Python to count the number of vowels in a given string. You should return the total number of vowels found.
  
  ### Test cases:
  assert count_vowels("Hello, World!") == 3
  assert count_vowels("Python") == 1
  assert count_vowels("AeIoU") == 5
  assert count_vowels("BCDFGHJKLMNPQRSTVWXYZ") == 0
  assert count_vowels("") == 0
output: def count_vowels(s):
      vowels = "aeiouAEIOU"
      count = 0
      for char in s:
          if char in vowels:
              count += 1
      return count