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'
      >>> compress("aabbcc")
      '2a2b2c'
      """
      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:
              output += str(count) + s[i]
              count = 1
      
      return output