input: def compress(s: str) -> str: 
      """ Compress a given string by replacing all repeating characters with a single instance and its count.
      >>> compress("aabbbbbbcccc")
      '2a6b4c'
      """
      output = "" 
      count = 1
      for i in range(len(s)): 
          if (i+1 < len(s) and s[i] == s[i+1]): 
              count += 1
          else: 
              output += str(count) + s[i] 
              count = 1
      output += str(count) + s[i] 
      return output
  
  s = "aabbbbbbcccc"
  print(compress(s))
  # Output: 2a6b4c
output:     output = "" 
      count = 1
      for i in range(len(s)): 
          if (i+1 < len(s) and s[i] == s[i+1]): 
              count += 1
          else: 
              output += str(count) + s[i] 
              count = 1
      return output