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