code: def read(cls, f):
      
      url = None
  
      line = f.readline()
      if not line:
        # EOF
        return None
      while not line.startswith(cls.LENGTH_HEADER):
        if line.startswith(cls.URI_HEADER):
          url = line[len(cls.URI_HEADER):].strip()
        line = f.readline()
  
      # Consume empty separator
      f.readline()
  
      # Read content
      length = int(line.split(':')[1])
  
      return cls(url, length)
docs: Read header from file. Headers end with length and then 1 blank line.