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